diff --git a/.github/workflows/php.yml b/.github/workflows/php.yml
index 71ab83c..d4cdbb6 100644
--- a/.github/workflows/php.yml
+++ b/.github/workflows/php.yml
@@ -80,12 +80,17 @@ jobs:
           name: tools
           path: tools
 
+      - name: Set tools as an executable
+        run: find tools -type f -print0 | xargs -0 chmod +x
+
       - name: Run lint
+        env:
+          PSALM_SHEPHERD: 1
+          # TODO: Remove it when psalm no longer tries to create a cache directory
+          #       https://github.com/vimeo/psalm/issues/4267
+          XDG_CACHE_HOME: /tmp
         run: composer lint
 
-      - name: Run bug analysis
-        run: composer findbugs
-
       - name: Run unit tests
         env:
           XDEBUG_MODE: coverage
diff --git a/.phive/phars.xml b/.phive/phars.xml
index 74cb369..acda6c4 100644
--- a/.phive/phars.xml
+++ b/.phive/phars.xml
@@ -1,10 +1,11 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <phive xmlns="https://phar.io/phive">
   <phar name="php-coveralls" version="^2.4.3" installed="2.4.3" location="./tools/php-coveralls" copy="false"/>
-  <phar name="php-cs-fixer" version="^2.18.2" installed="2.18.2" location="./tools/php-cs-fixer" copy="false"/>
+  <phar name="php-cs-fixer" version="^2.18.2" installed="2.18.4" location="./tools/php-cs-fixer" copy="false"/>
   <phar name="phpcbf" version="^3.5.8" installed="3.5.8" location="./tools/phpcbf" copy="false"/>
   <phar name="phpcs" version="^3.5.8" installed="3.5.8" location="./tools/phpcs" copy="false"/>
   <phar name="phpdocumentor" version="^3.0.0" installed="3.0.0" location="./tools/phpDocumentor" copy="false"/>
-  <phar name="phpstan" version="^0.12.64" installed="0.12.80" location="./tools/phpstan" copy="false"/>
-  <phar name="phpunit" version="^9.5.0" installed="9.5.2" location="./tools/phpunit" copy="false"/>
+  <phar name="phpstan" version="^0.12.64" installed="0.12.82" location="./tools/phpstan" copy="false"/>
+  <phar name="phpunit" version="^9.5.0" installed="9.5.4" location="./tools/phpunit" copy="false"/>
+  <phar name="psalm" version="^4.7.0" installed="4.7.0" location="./tools/psalm" copy="false"/>
 </phive>
diff --git a/.vim/coc-settings.json b/.vim/coc-settings.json
index b9254f6..a7ada7f 100644
--- a/.vim/coc-settings.json
+++ b/.vim/coc-settings.json
@@ -1,10 +1,13 @@
 {
     "diagnostic-languageserver.linters": {
-        "ngmy.laradock.composer.lint": {
-            "args": ["exec", "-u", "laradock", "laradock-ngmy-php-observer_workspace_1", "composer", "lint", "--", "--report=emacs"]
+        "ngmy.laradock.phive.phpcs": {
+            "args": ["exec", "-u", "laradock", "laradock-ngmy-php-observer_workspace_1", "php", "tools/phpcs", "--report=emacs"]
         },
-        "ngmy.laradock.composer.findbugs": {
-            "args": ["exec", "-u", "laradock", "laradock-ngmy-php-observer_workspace_1", "composer", "findbugs", "--", "--error-format", "raw", "--no-progress"]
+        "ngmy.laradock.phive.phpstan": {
+            "args": ["exec", "-u", "laradock", "laradock-ngmy-php-observer_workspace_1", "php", "tools/phpstan", "analyse", "--error-format", "raw", "--no-progress"]
+        },
+        "ngmy.laradock.phive.psalm": {
+            "args": ["exec", "-u", "laradock", "laradock-ngmy-php-observer_workspace_1", "php", "tools/psalm", "--output-format=emacs", "--no-progress", "--no-cache"]
         }
     }
 }
diff --git a/README.md b/README.md
index 940bcb6..085945d 100644
--- a/README.md
+++ b/README.md
@@ -7,6 +7,8 @@
 [![PHP CI](https://github.com/ngmy/php-observer/actions/workflows/php.yml/badge.svg)](https://github.com/ngmy/php-observer/actions/workflows/php.yml)
 [![Coverage Status](https://coveralls.io/repos/github/ngmy/php-observer/badge.svg?branch=master)](https://coveralls.io/github/ngmy/php-observer?branch=master)
 [![PHPStan](https://img.shields.io/badge/PHPStan-enabled-brightgreen.svg?style=flat)](https://github.com/phpstan/phpstan)
+[![Psalm Coverage](https://shepherd.dev/github/ngmy/php-observer/coverage.svg?)](https://shepherd.dev/github/ngmy/php-observer)
+[![Psalm Level](https://shepherd.dev/github/ngmy/php-observer/level.svg?)](https://shepherd.dev/github/ngmy/php-observer)
 
 PHP Observer is a PHP library to implement the [observer pattern](https://en.wikipedia.org/wiki/Observer_pattern).
 
diff --git a/composer.json b/composer.json
index 53ca541..d280150 100644
--- a/composer.json
+++ b/composer.json
@@ -43,20 +43,40 @@
     "prefer-stable": true,
     "scripts": {
         "test": [
-            "@php tools/phpunit --colors=always"
+            "@phpunit"
         ],
         "lint": [
-            "@php tools/phpcs"
-        ],
-        "findbugs": [
-            "@php tools/phpstan analyse --ansi"
+            "@phpcs",
+            "@phpstan",
+            "@psalm"
         ],
         "fmt": [
-            "@php tools/phpcbf | exit 0",
-            "@php tools/php-cs-fixer fix --ansi"
+            "@phpcbf",
+            "@php-cs-fixer"
+        ],
+        "doc": [
+            "@phpdoc"
+        ],
+        "php-cs-fixer": [
+            "tools/php-cs-fixer fix --ansi"
+        ],
+        "phpcbf": [
+            "tools/phpcbf"
+        ],
+        "phpcs": [
+            "tools/phpcs"
+        ],
+        "phpdoc": [
+            "tools/phpDocumentor --force"
+        ],
+        "phpstan": [
+            "tools/phpstan analyse --ansi"
+        ],
+        "phpunit": [
+            "tools/phpunit --colors=always"
         ],
-        "docs": [
-            "@php tools/phpDocumentor --force"
+        "psalm": [
+            "[ \"${PSALM_SHEPHERD:-0}\" = '0' ] && tools/psalm --no-cache || tools/psalm --no-cache --shepherd"
         ]
     }
 }
diff --git a/psalm.xml.dist b/psalm.xml.dist
new file mode 100644
index 0000000..3240886
--- /dev/null
+++ b/psalm.xml.dist
@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<psalm
+    errorLevel="1"
+    resolveFromConfigFile="true"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xmlns="https://getpsalm.org/schema/config"
+    xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"
+>
+    <projectFiles>
+        <directory name="src" />
+        <ignoreFiles>
+            <directory name="vendor" />
+        </ignoreFiles>
+    </projectFiles>
+</psalm>
diff --git a/src/Subject.php b/src/Subject.php
index 77cbf9a..b083555 100644
--- a/src/Subject.php
+++ b/src/Subject.php
@@ -9,6 +9,7 @@ abstract class Subject
     /**
      * @var array<int, Observer>
      * @phpstan-var list<Observer>
+     * @psalm-var list<Observer>
      */
     protected $observers = [];