diff --git a/lithic-java-core/src/main/kotlin/com/lithic/api/core/Utils.kt b/lithic-java-core/src/main/kotlin/com/lithic/api/core/Utils.kt
index 17e814ed9..91a458e13 100644
--- a/lithic-java-core/src/main/kotlin/com/lithic/api/core/Utils.kt
+++ b/lithic-java-core/src/main/kotlin/com/lithic/api/core/Utils.kt
@@ -6,6 +6,7 @@ import com.google.common.collect.ImmutableListMultimap
 import com.google.common.collect.ListMultimap
 import com.lithic.api.errors.LithicInvalidDataException
 import java.util.Collections
+import java.util.SortedMap
 
 @JvmSynthetic
 internal fun <T : Any> T?.getOrThrow(name: String): T =
@@ -19,6 +20,11 @@ internal fun <T> List<T>.toImmutable(): List<T> =
 internal fun <K, V> Map<K, V>.toImmutable(): Map<K, V> =
     if (isEmpty()) Collections.emptyMap() else Collections.unmodifiableMap(toMap())
 
+@JvmSynthetic
+internal fun <K : Comparable<K>, V> SortedMap<K, V>.toImmutable(): SortedMap<K, V> =
+    if (isEmpty()) Collections.emptySortedMap()
+    else Collections.unmodifiableSortedMap(toSortedMap(comparator()))
+
 @JvmSynthetic
 internal fun <K, V> ListMultimap<K, V>.toImmutable(): ListMultimap<K, V> =
     ImmutableListMultimap.copyOf(this)
diff --git a/lithic-java-core/src/main/kotlin/com/lithic/api/core/http/Headers.kt b/lithic-java-core/src/main/kotlin/com/lithic/api/core/http/Headers.kt
new file mode 100644
index 000000000..2c73be7e5
--- /dev/null
+++ b/lithic-java-core/src/main/kotlin/com/lithic/api/core/http/Headers.kt
@@ -0,0 +1,92 @@
+package com.lithic.api.core.http
+
+import com.lithic.api.core.toImmutable
+import java.util.TreeMap
+
+class Headers
+private constructor(
+    private val map: Map<String, List<String>>,
+    @get:JvmName("size") val size: Int
+) {
+
+    fun isEmpty(): Boolean = map.isEmpty()
+
+    fun names(): Set<String> = map.keys
+
+    fun values(name: String): List<String> = map[name].orEmpty()
+
+    fun toBuilder(): Builder = Builder().putAll(map)
+
+    companion object {
+
+        @JvmStatic fun builder() = Builder()
+    }
+
+    class Builder {
+
+        private val map: MutableMap<String, MutableList<String>> =
+            TreeMap(String.CASE_INSENSITIVE_ORDER)
+        private var size: Int = 0
+
+        fun put(name: String, value: String) = apply {
+            map.getOrPut(name) { mutableListOf() }.add(value)
+            size++
+        }
+
+        fun put(name: String, values: Iterable<String>) = apply { values.forEach { put(name, it) } }
+
+        fun putAll(headers: Map<String, Iterable<String>>) = apply { headers.forEach(::put) }
+
+        fun putAll(headers: Headers) = apply {
+            headers.names().forEach { put(it, headers.values(it)) }
+        }
+
+        fun remove(name: String) = apply { size -= map.remove(name).orEmpty().size }
+
+        fun removeAll(names: Set<String>) = apply { names.forEach(::remove) }
+
+        fun clear() = apply {
+            map.clear()
+            size = 0
+        }
+
+        fun replace(name: String, value: String) = apply {
+            remove(name)
+            put(name, value)
+        }
+
+        fun replace(name: String, values: Iterable<String>) = apply {
+            remove(name)
+            put(name, values)
+        }
+
+        fun replaceAll(headers: Map<String, Iterable<String>>) = apply {
+            headers.forEach(::replace)
+        }
+
+        fun replaceAll(headers: Headers) = apply {
+            headers.names().forEach { replace(it, headers.values(it)) }
+        }
+
+        fun build() =
+            Headers(
+                map.mapValuesTo(TreeMap(String.CASE_INSENSITIVE_ORDER)) { (_, values) ->
+                        values.toImmutable()
+                    }
+                    .toImmutable(),
+                size
+            )
+    }
+
+    override fun hashCode(): Int = map.hashCode()
+
+    override fun equals(other: Any?): Boolean {
+        if (this === other) {
+            return true
+        }
+
+        return other is Headers && map == other.map
+    }
+
+    override fun toString(): String = "Headers{map=$map}"
+}
diff --git a/lithic-java-core/src/test/kotlin/com/lithic/api/core/http/HeadersTest.kt b/lithic-java-core/src/test/kotlin/com/lithic/api/core/http/HeadersTest.kt
new file mode 100644
index 000000000..87fc845d0
--- /dev/null
+++ b/lithic-java-core/src/test/kotlin/com/lithic/api/core/http/HeadersTest.kt
@@ -0,0 +1,274 @@
+package com.lithic.api.core.http
+
+import org.assertj.core.api.Assertions.assertThat
+import org.assertj.core.api.Assertions.catchThrowable
+import org.assertj.core.api.Assumptions.assumeThat
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.EnumSource
+
+internal class HeadersTest {
+
+    enum class TestCase(
+        val headers: Headers,
+        val expectedMap: Map<String, List<String>>,
+        val expectedSize: Int
+    ) {
+        EMPTY(Headers.builder().build(), expectedMap = mapOf(), expectedSize = 0),
+        PUT_ONE(
+            Headers.builder().put("name", "value").build(),
+            expectedMap = mapOf("name" to listOf("value")),
+            expectedSize = 1
+        ),
+        PUT_MULTIPLE(
+            Headers.builder().put("name", listOf("value1", "value2")).build(),
+            expectedMap = mapOf("name" to listOf("value1", "value2")),
+            expectedSize = 2
+        ),
+        MULTIPLE_PUT(
+            Headers.builder().put("name1", "value").put("name2", "value").build(),
+            expectedMap = mapOf("name1" to listOf("value"), "name2" to listOf("value")),
+            expectedSize = 2
+        ),
+        MULTIPLE_PUT_SAME_NAME(
+            Headers.builder().put("name", "value1").put("name", "value2").build(),
+            expectedMap = mapOf("name" to listOf("value1", "value2")),
+            expectedSize = 2
+        ),
+        MULTIPLE_PUT_MULTIPLE(
+            Headers.builder()
+                .put("name", listOf("value1", "value2"))
+                .put("name", listOf("value1", "value2"))
+                .build(),
+            expectedMap = mapOf("name" to listOf("value1", "value2", "value1", "value2")),
+            expectedSize = 4
+        ),
+        PUT_CASE_INSENSITIVE(
+            Headers.builder()
+                .put("name", "value1")
+                .put("NAME", "value2")
+                .put("nAmE", "value3")
+                .build(),
+            expectedMap = mapOf("name" to listOf("value1", "value2", "value3")),
+            expectedSize = 3
+        ),
+        PUT_ALL_MAP(
+            Headers.builder()
+                .putAll(
+                    mapOf(
+                        "name1" to listOf("value1", "value2"),
+                        "name2" to listOf("value1", "value2")
+                    )
+                )
+                .build(),
+            expectedMap =
+                mapOf("name1" to listOf("value1", "value2"), "name2" to listOf("value1", "value2")),
+            expectedSize = 4
+        ),
+        PUT_ALL_HEADERS(
+            Headers.builder().putAll(Headers.builder().put("name", "value").build()).build(),
+            expectedMap = mapOf("name" to listOf("value")),
+            expectedSize = 1
+        ),
+        PUT_ALL_CASE_INSENSITIVE(
+            Headers.builder()
+                .putAll(
+                    mapOf(
+                        "name" to listOf("value1"),
+                        "NAME" to listOf("value2"),
+                        "nAmE" to listOf("value3")
+                    )
+                )
+                .build(),
+            expectedMap = mapOf("name" to listOf("value1", "value2", "value3")),
+            expectedSize = 3
+        ),
+        REMOVE_ABSENT(
+            Headers.builder().remove("name").build(),
+            expectedMap = mapOf(),
+            expectedSize = 0
+        ),
+        REMOVE_PRESENT_ONE(
+            Headers.builder().put("name", "value").remove("name").build(),
+            expectedMap = mapOf(),
+            expectedSize = 0
+        ),
+        REMOVE_PRESENT_MULTIPLE(
+            Headers.builder().put("name", listOf("value1", "value2")).remove("name").build(),
+            expectedMap = mapOf(),
+            expectedSize = 0
+        ),
+        REMOVE_CASE_INSENSITIVE(
+            Headers.builder().put("name", listOf("value1", "value2")).remove("NAME").build(),
+            expectedMap = mapOf(),
+            expectedSize = 0
+        ),
+        REMOVE_ALL(
+            Headers.builder()
+                .put("name1", "value")
+                .put("name3", "value")
+                .removeAll(setOf("name1", "name2", "name3"))
+                .build(),
+            expectedMap = mapOf(),
+            expectedSize = 0
+        ),
+        REMOVE_ALL_CASE_INSENSITIVE(
+            Headers.builder()
+                .put("name1", "value")
+                .put("name3", "value")
+                .removeAll(setOf("NAME1", "nAmE3"))
+                .build(),
+            expectedMap = mapOf(),
+            expectedSize = 0
+        ),
+        CLEAR(
+            Headers.builder().put("name1", "value").put("name2", "value").clear().build(),
+            expectedMap = mapOf(),
+            expectedSize = 0
+        ),
+        REPLACE_ONE_ABSENT(
+            Headers.builder().replace("name", "value").build(),
+            expectedMap = mapOf("name" to listOf("value")),
+            expectedSize = 1
+        ),
+        REPLACE_ONE_PRESENT_ONE(
+            Headers.builder().put("name", "value1").replace("name", "value2").build(),
+            expectedMap = mapOf("name" to listOf("value2")),
+            expectedSize = 1
+        ),
+        REPLACE_ONE_PRESENT_MULTIPLE(
+            Headers.builder()
+                .put("name", listOf("value1", "value2"))
+                .replace("name", "value3")
+                .build(),
+            expectedMap = mapOf("name" to listOf("value3")),
+            expectedSize = 1
+        ),
+        REPLACE_MULTIPLE_ABSENT(
+            Headers.builder().replace("name", listOf("value1", "value2")).build(),
+            expectedMap = mapOf("name" to listOf("value1", "value2")),
+            expectedSize = 2
+        ),
+        REPLACE_MULTIPLE_PRESENT_ONE(
+            Headers.builder()
+                .put("name", "value1")
+                .replace("name", listOf("value2", "value3"))
+                .build(),
+            expectedMap = mapOf("name" to listOf("value2", "value3")),
+            expectedSize = 2
+        ),
+        REPLACE_MULTIPLE_PRESENT_MULTIPLE(
+            Headers.builder()
+                .put("name", listOf("value1", "value2"))
+                .replace("name", listOf("value3", "value4"))
+                .build(),
+            expectedMap = mapOf("name" to listOf("value3", "value4")),
+            expectedSize = 2
+        ),
+        REPLACE_CASE_INSENSITIVE(
+            Headers.builder()
+                .put("name", "value1")
+                .replace("NAME", listOf("value2", "value3"))
+                .build(),
+            expectedMap = mapOf("NAME" to listOf("value2", "value3")),
+            expectedSize = 2
+        ),
+        REPLACE_ALL_MAP(
+            Headers.builder()
+                .put("name1", "value1")
+                .put("name2", "value1")
+                .put("name3", "value1")
+                .replaceAll(mapOf("name1" to listOf("value2"), "name3" to listOf("value2")))
+                .build(),
+            expectedMap =
+                mapOf(
+                    "name1" to listOf("value2"),
+                    "name2" to listOf("value1"),
+                    "name3" to listOf("value2")
+                ),
+            expectedSize = 3
+        ),
+        REPLACE_ALL_HEADERS(
+            Headers.builder()
+                .put("name1", "value1")
+                .put("name2", "value1")
+                .put("name3", "value1")
+                .replaceAll(Headers.builder().put("name1", "value2").put("name3", "value2").build())
+                .build(),
+            expectedMap =
+                mapOf(
+                    "name1" to listOf("value2"),
+                    "name2" to listOf("value1"),
+                    "name3" to listOf("value2")
+                ),
+            expectedSize = 3
+        ),
+        REPLACE_ALL_CASE_INSENSITIVE(
+            Headers.builder()
+                .put("name1", "value1")
+                .put("name2", "value1")
+                .replaceAll(mapOf("NAME1" to listOf("value2"), "nAmE2" to listOf("value2")))
+                .build(),
+            expectedMap = mapOf("NAME1" to listOf("value2"), "nAmE2" to listOf("value2")),
+            expectedSize = 2
+        )
+    }
+
+    @ParameterizedTest
+    @EnumSource
+    fun namesAndValues(testCase: TestCase) {
+        val map = mutableMapOf<String, List<String>>()
+        val headers = testCase.headers
+        headers.names().forEach { name -> map[name] = headers.values(name) }
+
+        assertThat(map).isEqualTo(testCase.expectedMap)
+    }
+
+    @ParameterizedTest
+    @EnumSource
+    fun caseInsensitiveNames(testCase: TestCase) {
+        val headers = testCase.headers
+
+        for (name in headers.names()) {
+            assertThat(headers.values(name)).isEqualTo(headers.values(name.lowercase()))
+            assertThat(headers.values(name)).isEqualTo(headers.values(name.uppercase()))
+        }
+    }
+
+    @ParameterizedTest
+    @EnumSource
+    fun size(testCase: TestCase) {
+        val size = testCase.headers.size
+
+        assertThat(size).isEqualTo(testCase.expectedSize)
+    }
+
+    @ParameterizedTest
+    @EnumSource
+    fun namesAreImmutable(testCase: TestCase) {
+        val headers = testCase.headers
+        val headerNamesCopy = headers.names().toSet()
+
+        val throwable = catchThrowable {
+            (headers.names() as MutableSet<String>).add("another name")
+        }
+
+        assertThat(throwable).isInstanceOf(UnsupportedOperationException::class.java)
+        assertThat(headers.names()).isEqualTo(headerNamesCopy)
+    }
+
+    @ParameterizedTest
+    @EnumSource
+    fun valuesAreImmutable(testCase: TestCase) {
+        val headers = testCase.headers
+        assumeThat(headers.size).isNotEqualTo(0)
+        val name = headers.names().first()
+        val headerValuesCopy = headers.values(name).toList()
+
+        val throwable = catchThrowable {
+            (headers.values(name) as MutableList<String>).add("another value")
+        }
+
+        assertThat(throwable).isInstanceOf(UnsupportedOperationException::class.java)
+        assertThat(headers.values(name)).isEqualTo(headerValuesCopy)
+    }
+}