diff --git a/src/Exception/InvalidMessageShapeException.php b/src/Exception/InvalidMessageShapeException.php
new file mode 100644
index 0000000..430ede4
--- /dev/null
+++ b/src/Exception/InvalidMessageShapeException.php
@@ -0,0 +1,35 @@
+<?php
+
+/**
+ * This file is part of skillshare/formatphp
+ *
+ * skillshare/formatphp is open source software: you can distribute
+ * it and/or modify it under the terms of the MIT License
+ * (the "License"). You may not use this file except in
+ * compliance with the License.
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+ * implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ *
+ * @copyright Copyright (c) Skillshare, Inc. <https://www.skillshare.com>
+ * @license https://opensource.org/licenses/MIT MIT License
+ */
+
+declare(strict_types=1);
+
+namespace FormatPHP\Exception;
+
+use FormatPHP\Reader\FormatInterface;
+use RuntimeException as PhpRuntimeException;
+
+/**
+ * Thrown when reading a message that doesn't conform to the expected shape
+ *
+ * @see FormatInterface
+ */
+class InvalidMessageShapeException extends PhpRuntimeException implements FormatPHPExceptionInterface
+{
+}
diff --git a/src/Exception/LocaleNotFoundException.php b/src/Exception/LocaleNotFoundException.php
new file mode 100644
index 0000000..d2604ac
--- /dev/null
+++ b/src/Exception/LocaleNotFoundException.php
@@ -0,0 +1,32 @@
+<?php
+
+/**
+ * This file is part of skillshare/formatphp
+ *
+ * skillshare/formatphp is open source software: you can distribute
+ * it and/or modify it under the terms of the MIT License
+ * (the "License"). You may not use this file except in
+ * compliance with the License.
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+ * implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ *
+ * @copyright Copyright (c) Skillshare, Inc. <https://www.skillshare.com>
+ * @license https://opensource.org/licenses/MIT MIT License
+ */
+
+declare(strict_types=1);
+
+namespace FormatPHP\Exception;
+
+use RuntimeException as PhpRuntimeException;
+
+/**
+ * Thrown when unable to find a given locale messages file
+ */
+class LocaleNotFoundException extends PhpRuntimeException implements FormatPHPExceptionInterface
+{
+}
diff --git a/src/MessageLoader.php b/src/MessageLoader.php
new file mode 100644
index 0000000..bc6231b
--- /dev/null
+++ b/src/MessageLoader.php
@@ -0,0 +1,141 @@
+<?php
+
+/**
+ * This file is part of skillshare/formatphp
+ *
+ * skillshare/formatphp is open source software: you can distribute
+ * it and/or modify it under the terms of the MIT License
+ * (the "License"). You may not use this file except in
+ * compliance with the License.
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+ * implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ *
+ * @copyright Copyright (c) Skillshare, Inc. <https://www.skillshare.com>
+ * @license https://opensource.org/licenses/MIT MIT License
+ */
+
+declare(strict_types=1);
+
+namespace FormatPHP;
+
+use FormatPHP\Exception\InvalidArgumentException;
+use FormatPHP\Exception\InvalidMessageShapeException;
+use FormatPHP\Exception\LocaleNotFoundException;
+use FormatPHP\Exception\UnableToProcessFileException;
+use FormatPHP\Intl\Locale;
+use FormatPHP\Intl\LocaleInterface;
+use FormatPHP\Reader\FormatInterface;
+use FormatPHP\Util\FileSystemHelper;
+
+use function array_filter;
+use function array_unique;
+use function array_values;
+use function implode;
+use function sprintf;
+
+use const DIRECTORY_SEPARATOR;
+
+/**
+ * Loads messages for a given locale from the file system or cache
+ */
+final class MessageLoader
+{
+    private Config $config;
+    private FileSystemHelper $fileSystemHelper;
+    private FormatInterface $formatReader;
+    private string $messagesDirectory;
+
+    /**
+     * @throws InvalidArgumentException
+     */
+    public function __construct(
+        string $messagesDirectory,
+        Config $config,
+        FormatInterface $formatReader,
+        ?FileSystemHelper $fileSystemHelper = null
+    ) {
+        $this->config = $config;
+        $this->formatReader = $formatReader;
+        $this->fileSystemHelper = $fileSystemHelper ?? new FileSystemHelper();
+        $this->messagesDirectory = $this->fileSystemHelper->getRealPath($messagesDirectory);
+
+        if (!$this->fileSystemHelper->isDirectory($this->messagesDirectory)) {
+            throw new InvalidArgumentException(sprintf(
+                'Messages directory "%s" is not a valid directory',
+                $messagesDirectory,
+            ));
+        }
+    }
+
+    /**
+     * Returns a MessageCollection according to the configuration provided to
+     * this MessageLoader
+     *
+     * @throws InvalidArgumentException
+     * @throws InvalidMessageShapeException
+     * @throws LocaleNotFoundException
+     */
+    public function loadMessages(): MessageCollection
+    {
+        [$messagesData, $resolvedLocale] = $this->getLocaleMessages();
+
+        return ($this->formatReader)($this->config, $messagesData, $resolvedLocale);
+    }
+
+    /**
+     * @return array{0: array<array-key, mixed>, 1: LocaleInterface}
+     *
+     * @throws InvalidArgumentException
+     * @throws LocaleNotFoundException
+     */
+    private function getLocaleMessages(): array
+    {
+        $messagesContents = false;
+        $localeResolved = null;
+
+        foreach ($this->getFallbackLocales() as $locale) {
+            try {
+                $messagesFile = $this->messagesDirectory . DIRECTORY_SEPARATOR . $locale . '.json';
+                $messagesContents = $this->fileSystemHelper->getJsonContents($messagesFile);
+                $localeResolved = new Locale($locale);
+
+                break;
+            } catch (UnableToProcessFileException $exception) {
+                continue;
+            }
+        }
+
+        if ($messagesContents === false || $localeResolved === null) {
+            throw new LocaleNotFoundException(sprintf(
+                'Unable to find a suitable locale for "%s"; please set a default locale',
+                $this->config->getLocale()->toString(),
+            ));
+        }
+
+        return [$messagesContents, $localeResolved];
+    }
+
+    /**
+     * @return string[]
+     */
+    private function getFallbackLocales(): array
+    {
+        $locale = $this->config->getLocale();
+        $defaultLocale = $this->config->getDefaultLocale();
+
+        $fallbacks = [
+            $locale->toString(),
+            $locale->baseName(),
+            implode('-', array_filter([$locale->language(), $locale->region()])),
+            $locale->language(),
+            $defaultLocale ? $defaultLocale->toString() : null,
+        ];
+
+        /** @var string[] */
+        return array_values(array_unique(array_filter($fallbacks)));
+    }
+}
diff --git a/src/Reader/Format/FormatPHP.php b/src/Reader/Format/FormatPHP.php
new file mode 100644
index 0000000..8b50e32
--- /dev/null
+++ b/src/Reader/Format/FormatPHP.php
@@ -0,0 +1,90 @@
+<?php
+
+/**
+ * This file is part of skillshare/formatphp
+ *
+ * skillshare/formatphp is open source software: you can distribute
+ * it and/or modify it under the terms of the MIT License
+ * (the "License"). You may not use this file except in
+ * compliance with the License.
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+ * implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ *
+ * @copyright Copyright (c) Skillshare, Inc. <https://www.skillshare.com>
+ * @license https://opensource.org/licenses/MIT MIT License
+ */
+
+declare(strict_types=1);
+
+namespace FormatPHP\Reader\Format;
+
+use FormatPHP\Config;
+use FormatPHP\Exception\InvalidMessageShapeException;
+use FormatPHP\Intl\LocaleInterface;
+use FormatPHP\Message;
+use FormatPHP\MessageCollection;
+use FormatPHP\Reader\FormatInterface;
+
+use function assert;
+use function gettype;
+use function is_array;
+use function is_string;
+use function sprintf;
+
+/**
+ * Returns a MessageCollection parsed from JSON-decoded data that was written
+ * using Writer\Format\FormatPHP
+ *
+ * @see \FormatPHP\Writer\Format\FormatPHP
+ */
+class FormatPHP implements FormatInterface
+{
+    /**
+     * @inheritdoc
+     */
+    public function __invoke(Config $config, array $data, LocaleInterface $localeResolved): MessageCollection
+    {
+        $messages = new MessageCollection($config);
+
+        foreach ($data as $messageId => $message) {
+            $this->validateShape($messageId, $message);
+            assert(is_string($messageId));
+            assert(isset($message['defaultMessage']));
+            assert(is_string($message['defaultMessage']));
+
+            $messages[] = new Message($localeResolved, $messageId, $message['defaultMessage']);
+        }
+
+        return $messages;
+    }
+
+    /**
+     * @param array-key $messageId
+     * @param mixed $message
+     *
+     * @throws InvalidMessageShapeException
+     *
+     * @phpcsSuppress SlevomatCodingStandard.TypeHints.ParameterTypeHint.MissingNativeTypeHint
+     */
+    private function validateShape($messageId, $message): void
+    {
+        if (!is_string($messageId)) {
+            throw new InvalidMessageShapeException(sprintf(
+                '%s expects a string message ID; received %s',
+                self::class,
+                gettype($messageId),
+            ));
+        }
+
+        if (!is_array($message) || !is_string($message['defaultMessage'] ?? null)) {
+            throw new InvalidMessageShapeException(sprintf(
+                '%s expects a string defaultMessage property; defaultMessage does not exist or is not a string',
+                self::class,
+            ));
+        }
+    }
+}
diff --git a/src/Reader/Format/Simple.php b/src/Reader/Format/Simple.php
new file mode 100644
index 0000000..03536f6
--- /dev/null
+++ b/src/Reader/Format/Simple.php
@@ -0,0 +1,89 @@
+<?php
+
+/**
+ * This file is part of skillshare/formatphp
+ *
+ * skillshare/formatphp is open source software: you can distribute
+ * it and/or modify it under the terms of the MIT License
+ * (the "License"). You may not use this file except in
+ * compliance with the License.
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+ * implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ *
+ * @copyright Copyright (c) Skillshare, Inc. <https://www.skillshare.com>
+ * @license https://opensource.org/licenses/MIT MIT License
+ */
+
+declare(strict_types=1);
+
+namespace FormatPHP\Reader\Format;
+
+use FormatPHP\Config;
+use FormatPHP\Exception\InvalidMessageShapeException;
+use FormatPHP\Intl\LocaleInterface;
+use FormatPHP\Message;
+use FormatPHP\MessageCollection;
+use FormatPHP\Reader\FormatInterface;
+
+use function assert;
+use function gettype;
+use function is_string;
+use function sprintf;
+
+/**
+ * Returns a MessageCollection parsed from JSON-decoded data that was written
+ * using Writer\Format\Simple
+ *
+ * @see \FormatPHP\Writer\Format\Simple
+ */
+class Simple implements FormatInterface
+{
+    /**
+     * @inheritdoc
+     */
+    public function __invoke(Config $config, array $data, LocaleInterface $localeResolved): MessageCollection
+    {
+        $messages = new MessageCollection($config);
+
+        foreach ($data as $messageId => $message) {
+            $this->validateShape($messageId, $message);
+            assert(is_string($messageId));
+            assert(is_string($message));
+
+            $messages[$messageId] = new Message($localeResolved, $messageId, $message);
+        }
+
+        return $messages;
+    }
+
+    /**
+     * @param array-key $messageId
+     * @param mixed $message
+     *
+     * @throws InvalidMessageShapeException
+     *
+     * @phpcsSuppress SlevomatCodingStandard.TypeHints.ParameterTypeHint.MissingNativeTypeHint
+     */
+    private function validateShape($messageId, $message): void
+    {
+        if (!is_string($messageId)) {
+            throw new InvalidMessageShapeException(sprintf(
+                '%s expects a string message ID; received %s',
+                self::class,
+                gettype($messageId),
+            ));
+        }
+
+        if (!is_string($message)) {
+            throw new InvalidMessageShapeException(sprintf(
+                '%s expects a string message; received %s',
+                self::class,
+                gettype($message),
+            ));
+        }
+    }
+}
diff --git a/src/Reader/Format/Smartling.php b/src/Reader/Format/Smartling.php
new file mode 100644
index 0000000..aad282a
--- /dev/null
+++ b/src/Reader/Format/Smartling.php
@@ -0,0 +1,92 @@
+<?php
+
+/**
+ * This file is part of skillshare/formatphp
+ *
+ * skillshare/formatphp is open source software: you can distribute
+ * it and/or modify it under the terms of the MIT License
+ * (the "License"). You may not use this file except in
+ * compliance with the License.
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+ * implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ *
+ * @copyright Copyright (c) Skillshare, Inc. <https://www.skillshare.com>
+ * @license https://opensource.org/licenses/MIT MIT License
+ */
+
+declare(strict_types=1);
+
+namespace FormatPHP\Reader\Format;
+
+use FormatPHP\Config;
+use FormatPHP\Exception\InvalidMessageShapeException;
+use FormatPHP\Intl\LocaleInterface;
+use FormatPHP\Message;
+use FormatPHP\MessageCollection;
+use FormatPHP\Reader\FormatInterface;
+
+use function assert;
+use function gettype;
+use function is_array;
+use function is_string;
+use function sprintf;
+
+/**
+ * Returns a MessageCollection parsed from JSON-decoded data that was written
+ * using Writer\Format\Smartling
+ *
+ * @see \FormatPHP\Writer\Format\Smartling
+ */
+class Smartling implements FormatInterface
+{
+    /**
+     * @inheritdoc
+     */
+    public function __invoke(Config $config, array $data, LocaleInterface $localeResolved): MessageCollection
+    {
+        $messages = new MessageCollection($config);
+
+        unset($data['smartling']);
+
+        foreach ($data as $messageId => $message) {
+            $this->validateShape($messageId, $message);
+            assert(is_string($messageId));
+            assert(isset($message['message']));
+            assert(is_string($message['message']));
+
+            $messages[] = new Message($localeResolved, $messageId, $message['message']);
+        }
+
+        return $messages;
+    }
+
+    /**
+     * @param array-key $messageId
+     * @param mixed $message
+     *
+     * @throws InvalidMessageShapeException
+     *
+     * @phpcsSuppress SlevomatCodingStandard.TypeHints.ParameterTypeHint.MissingNativeTypeHint
+     */
+    private function validateShape($messageId, $message): void
+    {
+        if (!is_string($messageId)) {
+            throw new InvalidMessageShapeException(sprintf(
+                '%s expects a string message ID; received %s',
+                self::class,
+                gettype($messageId),
+            ));
+        }
+
+        if (!is_array($message) || !is_string($message['message'] ?? null)) {
+            throw new InvalidMessageShapeException(sprintf(
+                '%s expects a string message property; message does not exist or is not a string',
+                self::class,
+            ));
+        }
+    }
+}
diff --git a/src/Reader/FormatInterface.php b/src/Reader/FormatInterface.php
new file mode 100644
index 0000000..97ec2dc
--- /dev/null
+++ b/src/Reader/FormatInterface.php
@@ -0,0 +1,47 @@
+<?php
+
+/**
+ * This file is part of skillshare/formatphp
+ *
+ * skillshare/formatphp is open source software: you can distribute
+ * it and/or modify it under the terms of the MIT License
+ * (the "License"). You may not use this file except in
+ * compliance with the License.
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+ * implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ *
+ * @copyright Copyright (c) Skillshare, Inc. <https://www.skillshare.com>
+ * @license https://opensource.org/licenses/MIT MIT License
+ */
+
+declare(strict_types=1);
+
+namespace FormatPHP\Reader;
+
+use FormatPHP\Config;
+use FormatPHP\Exception\InvalidMessageShapeException;
+use FormatPHP\Intl\LocaleInterface;
+use FormatPHP\MessageCollection;
+
+/**
+ * Returns a collection of messages parsed from JSON-decoded message data
+ */
+interface FormatInterface
+{
+    /**
+     * @param array<array-key, mixed> $data An arbitrary array of JSON-decoded
+     *     data, loaded from a message file.
+     * @param LocaleInterface $localeResolved We utilize a "fallback" algorithm
+     *     to look up a suitable replacement locale (i.e., if we receive "en-US"
+     *     and have only a locale for "en," we will use "en" instead). This
+     *     parameter is the actual locale we used, which may be different from
+     *     the one provided on Config.
+     *
+     * @throws InvalidMessageShapeException
+     */
+    public function __invoke(Config $config, array $data, LocaleInterface $localeResolved): MessageCollection;
+}
diff --git a/src/Util/FileSystemHelper.php b/src/Util/FileSystemHelper.php
index aa51cc8..2b2aa65 100644
--- a/src/Util/FileSystemHelper.php
+++ b/src/Util/FileSystemHelper.php
@@ -27,6 +27,7 @@
 use FormatPHP\Exception\InvalidArgumentException;
 use FormatPHP\Exception\UnableToProcessFileException;
 use FormatPHP\Exception\UnableToWriteFileException;
+use JsonException;
 
 use function file_get_contents;
 use function file_put_contents;
@@ -39,9 +40,14 @@
 use function is_readable;
 use function is_resource;
 use function is_string;
+use function json_decode;
+use function realpath;
 use function sprintf;
 use function strlen;
 
+use const JSON_BIGINT_AS_STRING;
+use const JSON_INVALID_UTF8_SUBSTITUTE;
+use const JSON_THROW_ON_ERROR;
 use const PHP_SAPI;
 
 /**
@@ -49,6 +55,8 @@
  */
 class FileSystemHelper
 {
+    private const JSON_DECODE_FLAGS = JSON_BIGINT_AS_STRING | JSON_INVALID_UTF8_SUBSTITUTE | JSON_THROW_ON_ERROR;
+
     private static ?string $currentWorkingDir = null;
 
     /**
@@ -78,6 +86,27 @@ public function getContents(string $filePath): string
         return $contents;
     }
 
+    /**
+     * @return array<array-key, mixed>
+     *
+     * @throws UnableToProcessFileException
+     */
+    public function getJsonContents(string $filePath): array
+    {
+        $contents = $this->getContents($filePath);
+
+        try {
+            /** @var array<array-key, mixed> */
+            return @json_decode($contents, true, 512, self::JSON_DECODE_FLAGS);
+        } catch (JsonException $exception) {
+            throw new UnableToProcessFileException(
+                sprintf('Unable to decode the JSON in the file "%s"', $filePath),
+                (int) $exception->getCode(),
+                $exception,
+            );
+        }
+    }
+
     /**
      * @param string | resource | mixed $file
      *
@@ -110,6 +139,23 @@ public function writeContents($file, string $contents): void
         }
     }
 
+    /**
+     * @throws InvalidArgumentException
+     */
+    public function getRealPath(string $path): string
+    {
+        $realpath = @realpath($path);
+
+        if ($realpath === false) {
+            throw new InvalidArgumentException(sprintf(
+                'Unable to find or access the path at "%s"',
+                $path,
+            ));
+        }
+
+        return $realpath;
+    }
+
     /**
      * Returns `true` if the path is a directory
      */
diff --git a/tests/MessageLoaderTest.php b/tests/MessageLoaderTest.php
new file mode 100644
index 0000000..f54ef6f
--- /dev/null
+++ b/tests/MessageLoaderTest.php
@@ -0,0 +1,96 @@
+<?php
+
+declare(strict_types=1);
+
+namespace FormatPHP\Test;
+
+use FormatPHP\Config;
+use FormatPHP\Exception\InvalidArgumentException;
+use FormatPHP\Exception\LocaleNotFoundException;
+use FormatPHP\Intl\Locale;
+use FormatPHP\MessageInterface;
+use FormatPHP\MessageLoader;
+use FormatPHP\Reader\Format\FormatPHP;
+use FormatPHP\Reader\FormatInterface;
+
+use function sprintf;
+
+class MessageLoaderTest extends TestCase
+{
+    public function testExceptionWhenDirectoryIsNotValid(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage(sprintf(
+            'Messages directory "%s" is not a valid directory',
+            __FILE__,
+        ));
+
+        new MessageLoader(
+            __FILE__,
+            new Config(new Locale('en')),
+            $this->mockery(FormatInterface::class),
+        );
+    }
+
+    public function testExceptionWhenUnableToFindSuitableLocale(): void
+    {
+        // Esperanto, Latin script, US region.
+        $locale = new Locale('eo-Latn-US');
+
+        $loader = new MessageLoader(
+            __DIR__ . '/fixtures/locales',
+            new Config($locale),
+            $this->mockery(FormatInterface::class),
+        );
+
+        $this->expectException(LocaleNotFoundException::class);
+        $this->expectExceptionMessage('Unable to find a suitable locale for "eo-Latn-US"; please set a default locale');
+
+        $loader->loadMessages();
+    }
+
+    public function testLoadMessagesFallsBackToDefaultLocale(): void
+    {
+        // Esperanto, Latin script, US region.
+        $locale = new Locale('eo-Latn-US');
+
+        $defaultLocale = new Locale('en');
+
+        $loader = new MessageLoader(
+            __DIR__ . '/fixtures/locales',
+            new Config($locale, $defaultLocale),
+            new FormatPHP(),
+        );
+
+        $collection = $loader->loadMessages();
+
+        $this->assertCount(1, $collection);
+        $this->assertNotNull($collection['about.inspire']);
+        $this->assertInstanceOf(MessageInterface::class, $collection['about.inspire']);
+        $this->assertSame('en', $collection['about.inspire']->getLocale()->toString());
+    }
+
+    public function testLoadMessagesWithFallback(): void
+    {
+        $locale = new Locale('ar-Arab-AE-VARIANT1');
+
+        $defaultLocale = new Locale('en');
+
+        $loader = new MessageLoader(
+            __DIR__ . '/fixtures/locales',
+            new Config($locale, $defaultLocale),
+            new FormatPHP(),
+        );
+
+        $collection = $loader->loadMessages();
+
+        $this->assertCount(1, $collection);
+        $this->assertNotNull($collection['about.inspire']);
+        $this->assertInstanceOf(MessageInterface::class, $collection['about.inspire']);
+        $this->assertSame('ar', $collection['about.inspire']->getLocale()->toString());
+        $this->assertSame(
+            'في Skillshare ، نقوم بتمكين الأعضاء للحصول على الإلهام.',
+            $collection['about.inspire']->getMessage(),
+        );
+    }
+}
diff --git a/tests/Reader/Format/FormatPHPTest.php b/tests/Reader/Format/FormatPHPTest.php
new file mode 100644
index 0000000..e017a53
--- /dev/null
+++ b/tests/Reader/Format/FormatPHPTest.php
@@ -0,0 +1,70 @@
+<?php
+
+declare(strict_types=1);
+
+namespace FormatPHP\Test\Reader\Format;
+
+use FormatPHP\Config;
+use FormatPHP\Exception\InvalidMessageShapeException;
+use FormatPHP\Intl\Locale;
+use FormatPHP\MessageCollection;
+use FormatPHP\MessageInterface;
+use FormatPHP\Reader\Format\FormatPHP;
+use FormatPHP\Test\TestCase;
+
+use function sprintf;
+
+class FormatPHPTest extends TestCase
+{
+    public function testThrowsExceptionWhenMessageIdIsNotAString(): void
+    {
+        $locale = new Locale('en');
+        $config = new Config($locale);
+        $formatReader = new FormatPHP();
+        $data = ['foo'];
+
+        $this->expectException(InvalidMessageShapeException::class);
+        $this->expectExceptionMessage(sprintf(
+            '%s expects a string message ID; received integer',
+            FormatPHP::class,
+        ));
+
+        $formatReader($config, $data, $locale);
+    }
+
+    public function testThrowsExceptionWhenMessageIsNotAString(): void
+    {
+        $locale = new Locale('en');
+        $config = new Config($locale);
+        $formatReader = new FormatPHP();
+        $data = ['foo' => ['bar']];
+
+        $this->expectException(InvalidMessageShapeException::class);
+        $this->expectExceptionMessage(sprintf(
+            '%s expects a string defaultMessage property; defaultMessage does not exist or is not a string',
+            FormatPHP::class,
+        ));
+
+        $formatReader($config, $data, $locale);
+    }
+
+    public function testInvoke(): void
+    {
+        $locale = new Locale('en-US');
+        $localeResolved = new Locale('en');
+        $config = new Config($locale);
+        $formatReader = new FormatPHP();
+        $data = ['foo' => ['defaultMessage' => 'I am foo'], 'bar' => ['defaultMessage' => 'I am bar']];
+
+        $collection = $formatReader($config, $data, $localeResolved);
+
+        $this->assertInstanceOf(MessageCollection::class, $collection);
+        $this->assertCount(2, $collection);
+        $this->assertInstanceOf(MessageInterface::class, $collection['foo']);
+        $this->assertSame('I am foo', $collection['foo']->getMessage());
+        $this->assertSame($localeResolved, $collection['foo']->getLocale());
+        $this->assertInstanceOf(MessageInterface::class, $collection['bar']);
+        $this->assertSame('I am bar', $collection['bar']->getMessage());
+        $this->assertSame($localeResolved, $collection['bar']->getLocale());
+    }
+}
diff --git a/tests/Reader/Format/SimpleTest.php b/tests/Reader/Format/SimpleTest.php
new file mode 100644
index 0000000..c802f46
--- /dev/null
+++ b/tests/Reader/Format/SimpleTest.php
@@ -0,0 +1,70 @@
+<?php
+
+declare(strict_types=1);
+
+namespace FormatPHP\Test\Reader\Format;
+
+use FormatPHP\Config;
+use FormatPHP\Exception\InvalidMessageShapeException;
+use FormatPHP\Intl\Locale;
+use FormatPHP\MessageCollection;
+use FormatPHP\MessageInterface;
+use FormatPHP\Reader\Format\Simple;
+use FormatPHP\Test\TestCase;
+
+use function sprintf;
+
+class SimpleTest extends TestCase
+{
+    public function testThrowsExceptionWhenMessageIdIsNotAString(): void
+    {
+        $locale = new Locale('en');
+        $config = new Config($locale);
+        $formatReader = new Simple();
+        $data = ['foo'];
+
+        $this->expectException(InvalidMessageShapeException::class);
+        $this->expectExceptionMessage(sprintf(
+            '%s expects a string message ID; received integer',
+            Simple::class,
+        ));
+
+        $formatReader($config, $data, $locale);
+    }
+
+    public function testThrowsExceptionWhenMessageIsNotAString(): void
+    {
+        $locale = new Locale('en');
+        $config = new Config($locale);
+        $formatReader = new Simple();
+        $data = ['foo' => ['bar']];
+
+        $this->expectException(InvalidMessageShapeException::class);
+        $this->expectExceptionMessage(sprintf(
+            '%s expects a string message; received array',
+            Simple::class,
+        ));
+
+        $formatReader($config, $data, $locale);
+    }
+
+    public function testInvoke(): void
+    {
+        $locale = new Locale('en-US');
+        $localeResolved = new Locale('en');
+        $config = new Config($locale);
+        $formatReader = new Simple();
+        $data = ['foo' => 'I am foo', 'bar' => 'I am bar'];
+
+        $collection = $formatReader($config, $data, $localeResolved);
+
+        $this->assertInstanceOf(MessageCollection::class, $collection);
+        $this->assertCount(2, $collection);
+        $this->assertInstanceOf(MessageInterface::class, $collection['foo']);
+        $this->assertSame('I am foo', $collection['foo']->getMessage());
+        $this->assertSame($localeResolved, $collection['foo']->getLocale());
+        $this->assertInstanceOf(MessageInterface::class, $collection['bar']);
+        $this->assertSame('I am bar', $collection['bar']->getMessage());
+        $this->assertSame($localeResolved, $collection['bar']->getLocale());
+    }
+}
diff --git a/tests/Reader/Format/SmartlingTest.php b/tests/Reader/Format/SmartlingTest.php
new file mode 100644
index 0000000..ae6091b
--- /dev/null
+++ b/tests/Reader/Format/SmartlingTest.php
@@ -0,0 +1,70 @@
+<?php
+
+declare(strict_types=1);
+
+namespace FormatPHP\Test\Reader\Format;
+
+use FormatPHP\Config;
+use FormatPHP\Exception\InvalidMessageShapeException;
+use FormatPHP\Intl\Locale;
+use FormatPHP\MessageCollection;
+use FormatPHP\MessageInterface;
+use FormatPHP\Reader\Format\Smartling;
+use FormatPHP\Test\TestCase;
+
+use function sprintf;
+
+class SmartlingTest extends TestCase
+{
+    public function testThrowsExceptionWhenMessageIdIsNotAString(): void
+    {
+        $locale = new Locale('en');
+        $config = new Config($locale);
+        $formatReader = new Smartling();
+        $data = ['smartling' => [], 'foo'];
+
+        $this->expectException(InvalidMessageShapeException::class);
+        $this->expectExceptionMessage(sprintf(
+            '%s expects a string message ID; received integer',
+            Smartling::class,
+        ));
+
+        $formatReader($config, $data, $locale);
+    }
+
+    public function testThrowsExceptionWhenMessageIsNotAString(): void
+    {
+        $locale = new Locale('en');
+        $config = new Config($locale);
+        $formatReader = new Smartling();
+        $data = ['smartling' => [], 'foo' => ['bar']];
+
+        $this->expectException(InvalidMessageShapeException::class);
+        $this->expectExceptionMessage(sprintf(
+            '%s expects a string message property; message does not exist or is not a string',
+            Smartling::class,
+        ));
+
+        $formatReader($config, $data, $locale);
+    }
+
+    public function testInvoke(): void
+    {
+        $locale = new Locale('en-US');
+        $localeResolved = new Locale('en');
+        $config = new Config($locale);
+        $formatReader = new Smartling();
+        $data = ['smartling' => [], 'foo' => ['message' => 'I am foo'], 'bar' => ['message' => 'I am bar']];
+
+        $collection = $formatReader($config, $data, $localeResolved);
+
+        $this->assertInstanceOf(MessageCollection::class, $collection);
+        $this->assertCount(2, $collection);
+        $this->assertInstanceOf(MessageInterface::class, $collection['foo']);
+        $this->assertSame('I am foo', $collection['foo']->getMessage());
+        $this->assertSame($localeResolved, $collection['foo']->getLocale());
+        $this->assertInstanceOf(MessageInterface::class, $collection['bar']);
+        $this->assertSame('I am bar', $collection['bar']->getMessage());
+        $this->assertSame($localeResolved, $collection['bar']->getLocale());
+    }
+}
diff --git a/tests/Util/FileSystemHelperTest.php b/tests/Util/FileSystemHelperTest.php
index 60a2193..3334109 100644
--- a/tests/Util/FileSystemHelperTest.php
+++ b/tests/Util/FileSystemHelperTest.php
@@ -20,6 +20,7 @@
 use function getcwd;
 use function is_resource;
 use function is_writable;
+use function realpath;
 use function sprintf;
 use function sys_get_temp_dir;
 use function tempnam;
@@ -236,4 +237,44 @@ public function testWriteContentsToFile(): void
 
         unlink($tmpFile);
     }
+
+    public function testGetRealPathThrowsExceptionWhenUnableToAccessFile(): void
+    {
+        $helper = new FileSystemHelper();
+
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Unable to find or access the path at "foo/bar"');
+
+        $helper->getRealPath('foo/bar');
+    }
+
+    public function testGetRealPath(): void
+    {
+        $helper = new FileSystemHelper();
+
+        $this->assertSame(realpath('.'), $helper->getRealPath('.'));
+    }
+
+    public function testGetJsonContentsThrowsExceptionWhenUnableToParseJson(): void
+    {
+        $helper = new FileSystemHelper();
+
+        $this->expectException(UnableToProcessFileException::class);
+        $this->expectExceptionMessage(sprintf(
+            'Unable to decode the JSON in the file "%s"',
+            __DIR__ . '/fixtures/get-json-contents-01.json',
+        ));
+
+        $helper->getJsonContents(__DIR__ . '/fixtures/get-json-contents-01.json');
+    }
+
+    public function testGetJsonContents(): void
+    {
+        $helper = new FileSystemHelper();
+
+        $this->assertSame(
+            ['foo' => 'bar'],
+            $helper->getJsonContents(__DIR__ . '/fixtures/get-json-contents-02.json'),
+        );
+    }
 }
diff --git a/tests/Util/fixtures/get-json-contents-01.json b/tests/Util/fixtures/get-json-contents-01.json
new file mode 100644
index 0000000..0ed10f5
--- /dev/null
+++ b/tests/Util/fixtures/get-json-contents-01.json
@@ -0,0 +1 @@
+this is not a json file
diff --git a/tests/Util/fixtures/get-json-contents-02.json b/tests/Util/fixtures/get-json-contents-02.json
new file mode 100644
index 0000000..e63d37b
--- /dev/null
+++ b/tests/Util/fixtures/get-json-contents-02.json
@@ -0,0 +1,3 @@
+{
+    "foo": "bar"
+}
diff --git a/tests/fixtures/locales/ar.json b/tests/fixtures/locales/ar.json
new file mode 100644
index 0000000..225ccb2
--- /dev/null
+++ b/tests/fixtures/locales/ar.json
@@ -0,0 +1,6 @@
+{
+    "about.inspire": {
+        "defaultMessage": "في Skillshare ، نقوم بتمكين الأعضاء للحصول على الإلهام.",
+        "description": "Marketing text found at https://skillshare.com/about"
+    }
+}
diff --git a/tests/fixtures/locales/en.json b/tests/fixtures/locales/en.json
new file mode 100644
index 0000000..bca9b21
--- /dev/null
+++ b/tests/fixtures/locales/en.json
@@ -0,0 +1,6 @@
+{
+    "about.inspire": {
+        "defaultMessage": "At Skillshare, we empower members to get inspired.",
+        "description": "Marketing text found at https://skillshare.com/about"
+    }
+}