diff --git a/index.d.ts b/index.d.ts
index 78ecc807..ee07d85a 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -147,6 +147,8 @@ export type {NonEmptyTuple} from './source/non-empty-tuple.d.ts';
 export type {FindGlobalInstanceType, FindGlobalType} from './source/find-global-type.d.ts';
 export type {If} from './source/if.d.ts';
 export type {IsUnion} from './source/is-union.d.ts';
+export type {IsLowercase} from './source/is-lowercase.d.ts';
+export type {IsUppercase} from './source/is-uppercase.d.ts';
 
 // Template literal types
 export type {CamelCase} from './source/camel-case.d.ts';
diff --git a/readme.md b/readme.md
index 13fb122b..0756ff4e 100644
--- a/readme.md
+++ b/readme.md
@@ -228,6 +228,8 @@ Click the type names for complete docs.
 - [`IsNull`](source/is-null.d.ts) - Returns a boolean for whether the given type is `null`.
 - [`IsTuple`](source/is-tuple.d.ts) - Returns a boolean for whether the given array is a tuple.
 - [`IsUnion`](source/is-union.d.ts) - Returns a boolean for whether the given type is a union.
+- [`IsLowercase`](source/is-lowercase.d.ts) - Returns a boolean for whether the given string literal is lowercase.
+- [`IsUppercase`](source/is-uppercase.d.ts) - Returns a boolean for whether the given string literal is uppercase.
 
 ### JSON
 
diff --git a/source/internal/array.d.ts b/source/internal/array.d.ts
index fb8b0f61..782e1979 100644
--- a/source/internal/array.d.ts
+++ b/source/internal/array.d.ts
@@ -93,3 +93,31 @@ T extends readonly [...infer U] ?
 Returns whether the given array `T` is readonly.
 */
 export type IsArrayReadonly<T extends UnknownArray> = If<IsNever<T>, false, T extends unknown[] ? false : true>;
+
+/**
+Returns a boolean for whether every element in an array type extends another type.
+
+Note: This type is not designed to be used with non-tuple arrays (like `number[]`), tuples with optional elements (like `[1?, 2?, 3?]`), or tuples that contain a rest element (like `[1, 2, ...number[]]`).
+
+@example
+```
+import type {Every} from 'type-fest';
+
+type A = Every<[1, 2, 3], number>;
+//=> true
+
+type B = Every<[1, 2, '3'], number>;
+//=> false
+
+type C = Every<[number, number | string], number>;
+//=> boolean
+
+type D = Every<[true, boolean, true], true>;
+//=> boolean
+```
+*/
+export type Every<TArray extends UnknownArray, Type> = TArray extends readonly [infer First, ...infer Rest]
+	? First extends Type
+		? Every<Rest, Type>
+		: false
+	: true;
diff --git a/source/internal/string.d.ts b/source/internal/string.d.ts
index b82832e1..b7b2e790 100644
--- a/source/internal/string.d.ts
+++ b/source/internal/string.d.ts
@@ -112,16 +112,6 @@ export type StringLength<S extends string> = string extends S
 	? never
 	: StringToArray<S>['length'];
 
-/**
-Returns a boolean for whether the string is lowercased.
-*/
-export type IsLowerCase<T extends string> = T extends Lowercase<T> ? true : false;
-
-/**
-Returns a boolean for whether the string is uppercased.
-*/
-export type IsUpperCase<T extends string> = T extends Uppercase<T> ? true : false;
-
 /**
 Returns a boolean for whether a string is whitespace.
 */
diff --git a/source/is-lowercase.d.ts b/source/is-lowercase.d.ts
new file mode 100644
index 00000000..01aa9113
--- /dev/null
+++ b/source/is-lowercase.d.ts
@@ -0,0 +1,36 @@
+import type {Every} from './internal/array.js';
+
+/**
+Returns a boolean for whether the given string literal is lowercase.
+
+@example
+```
+import type {IsLowercase} from 'type-fest';
+
+IsLowercase<'abc'>;
+//=> true
+
+IsLowercase<'Abc'>;
+//=> false
+
+IsLowercase<string>;
+//=> boolean
+```
+*/
+export type IsLowercase<S extends string> = Every<_IsLowercase<S>, true>;
+
+/**
+Loops through each part in the string and returns a boolean array indicating whether each part is lowercase.
+*/
+type _IsLowercase<S extends string, Accumulator extends boolean[] = []> = S extends `${infer First}${infer Rest}`
+	? _IsLowercase<Rest, [...Accumulator, IsLowercaseHelper<First>]>
+	: [...Accumulator, IsLowercaseHelper<S>];
+
+/**
+Returns a boolean for whether an individual part of the string is lowercase.
+*/
+type IsLowercaseHelper<S extends string> = S extends Lowercase<string>
+	? true
+	: S extends Uppercase<string> | Capitalize<string> | `${string}${Uppercase<string>}${string}`
+		? false
+		: boolean;
diff --git a/source/is-uppercase.d.ts b/source/is-uppercase.d.ts
new file mode 100644
index 00000000..218a5f0d
--- /dev/null
+++ b/source/is-uppercase.d.ts
@@ -0,0 +1,36 @@
+import type {Every} from './internal/array.js';
+
+/**
+Returns a boolean for whether the given string literal is uppercase.
+
+@example
+```
+import type {IsUppercase} from 'type-fest';
+
+IsUppercase<'ABC'>;
+//=> true
+
+IsUppercase<'Abc'>;
+//=> false
+
+IsUppercase<string>;
+//=> boolean
+```
+*/
+export type IsUppercase<S extends string> = Every<_IsUppercase<S>, true>;
+
+/**
+Loops through each part in the string and returns a boolean array indicating whether each part is uppercase.
+*/
+type _IsUppercase<S extends string, Accumulator extends boolean[] = []> = S extends `${infer First}${infer Rest}`
+	? _IsUppercase<Rest, [...Accumulator, IsUppercaseHelper<First>]>
+	: [...Accumulator, IsUppercaseHelper<S>];
+
+/**
+Returns a boolean for whether an individual part of the string is uppercase.
+*/
+type IsUppercaseHelper<S extends string> = S extends Uppercase<string>
+	? true
+	: S extends Lowercase<string> | Uncapitalize<string> | `${string}${Lowercase<string>}${string}`
+		? false
+		: boolean;
diff --git a/source/words.d.ts b/source/words.d.ts
index 0355d89a..fc15e14f 100644
--- a/source/words.d.ts
+++ b/source/words.d.ts
@@ -1,10 +1,10 @@
 import type {
 	ApplyDefaultOptions,
-	IsLowerCase,
 	IsNumeric,
-	IsUpperCase,
 	WordSeparators,
 } from './internal/index.d.ts';
+import type {IsLowercase} from './is-lowercase.d.ts';
+import type {IsUppercase} from './is-uppercase.d.ts';
 
 type SkipEmptyWord<Word extends string> = Word extends '' ? [] : [Word];
 
@@ -108,10 +108,10 @@ type WordsImplementation<
 					: [true, true] extends [IsNumeric<LastCharacter>, IsNumeric<FirstCharacter>]
 						? WordsImplementation<RemainingCharacters, Options, FirstCharacter, `${CurrentWord}${FirstCharacter}`>
 					// Case change: lower to upper, push word
-						: [true, true] extends [IsLowerCase<LastCharacter>, IsUpperCase<FirstCharacter>]
+						: [true, true] extends [IsLowercase<LastCharacter>, IsUppercase<FirstCharacter>]
 							? [...SkipEmptyWord<CurrentWord>, ...WordsImplementation<RemainingCharacters, Options, FirstCharacter, FirstCharacter>]
 						// Case change: upper to lower, brings back the last character, push word
-							: [true, true] extends [IsUpperCase<LastCharacter>, IsLowerCase<FirstCharacter>]
+							: [true, true] extends [IsUppercase<LastCharacter>, IsLowercase<FirstCharacter>]
 								? [...RemoveLastCharacter<CurrentWord, LastCharacter>, ...WordsImplementation<RemainingCharacters, Options, FirstCharacter, `${LastCharacter}${FirstCharacter}`>]
 							// No case change: concat word
 								: WordsImplementation<RemainingCharacters, Options, FirstCharacter, `${CurrentWord}${FirstCharacter}`>
diff --git a/test-d/internal/every.ts b/test-d/internal/every.ts
new file mode 100644
index 00000000..b8fa7102
--- /dev/null
+++ b/test-d/internal/every.ts
@@ -0,0 +1,31 @@
+import {expectType} from 'tsd';
+import type {Every} from '../../source/internal/array.js';
+
+expectType<Every<[], number>>(true);
+expectType<Every<[1, 2, 3], number>>(true);
+expectType<Every<[1, 2, 3], number | bigint>>(true);
+expectType<Every<[true, false, true, false, boolean], boolean>>(true);
+expectType<Every<[string, '1', '2', `${number}`], string>>(true);
+
+expectType<Every<[1, 2, '3'], number>>(false);
+expectType<Every<['1', 2, 3], number>>(false);
+expectType<Every<['1', '2', '3', 4, '5'], string>>(false);
+
+// Union type elements
+expectType<Every<[string, string | number], string>>({} as boolean);
+expectType<Every<[true, true, true, boolean], true>>({} as boolean);
+expectType<Every<[false, false, boolean, false], false>>({} as boolean);
+expectType<Every<['1', '2', number | bigint, '3'], string>>(false);
+expectType<Every<[1, 2, number | bigint, 3], number | bigint>>(true);
+
+// Readonly arrays
+expectType<Every<readonly [], number>>(true);
+expectType<Every<readonly [1, 2, 3], number>>(true);
+expectType<Every<readonly [1, 2, '3'], number>>(false);
+
+// Unions
+expectType<Every<[1, 2, 3] | [4, 5, 6], number>>(true); // Both `true`
+expectType<Every<[1, 2, '3'] | [4, 5, '6'], number>>(false); // Both `false`
+expectType<Every<[1, 2, 3] | ['1', '2', 3], number>>({} as boolean); // One `true`, one `false`
+expectType<Every<[true, true] | [true, boolean], true>>({} as boolean); // One `true`, one `boolean`
+expectType<Every<[true, false] | [true, boolean], true>>({} as boolean); // One `false`, one `boolean`
diff --git a/test-d/is-lowercase.ts b/test-d/is-lowercase.ts
new file mode 100644
index 00000000..ef355d51
--- /dev/null
+++ b/test-d/is-lowercase.ts
@@ -0,0 +1,56 @@
+import {expectType} from 'tsd';
+import type {IsLowercase} from '../index.d.ts';
+
+// Literals
+expectType<IsLowercase<'abc'>>(true);
+expectType<IsLowercase<''>>(true);
+expectType<IsLowercase<'123'>>(true);
+expectType<IsLowercase<'!@#'>>(true);
+
+expectType<IsLowercase<'Abc'>>(false);
+expectType<IsLowercase<'ABC'>>(false);
+
+// Non-literals
+expectType<IsLowercase<Lowercase<string>>>(true);
+expectType<IsLowercase<Lowercase<`on${string}`>>>(true);
+
+expectType<IsLowercase<Uppercase<string>>>(false);
+expectType<IsLowercase<Capitalize<`on${string}`>>>(false);
+expectType<IsLowercase<`on${Capitalize<string>}`>>(false);
+expectType<IsLowercase<Capitalize<`${string}end`>>>(false);
+
+expectType<IsLowercase<string>>({} as boolean);
+expectType<IsLowercase<`on${string}`>>({} as boolean);
+expectType<IsLowercase<Uncapitalize<string>>>({} as boolean);
+expectType<IsLowercase<Uncapitalize<`on${string}`>>>({} as boolean);
+expectType<IsLowercase<`on${Uncapitalize<string>}`>>({} as boolean);
+expectType<IsLowercase<`${number}`>>({} as boolean);
+
+expectType<IsLowercase<`Start${string}`>>(false);
+expectType<IsLowercase<`${string}End`>>(false);
+expectType<IsLowercase<`${string}Mid${string}`>>(false);
+
+// Complex non-literals
+expectType<IsLowercase<`${Lowercase<string>}${Lowercase<string>}`>>(true);
+
+expectType<IsLowercase<`${Uppercase<string>}${Lowercase<string>}`>>(false);
+expectType<IsLowercase<`${Lowercase<string>}${Uppercase<string>}`>>(false);
+expectType<IsLowercase<`${Lowercase<string>}${Uppercase<string>}${Lowercase<string>}`>>(false);
+expectType<IsLowercase<`${Capitalize<string>}${Lowercase<string>}`>>(false);
+expectType<IsLowercase<`${Lowercase<string>}${Capitalize<string>}`>>(false);
+expectType<IsLowercase<`${string}${Capitalize<string>}`>>(false);
+expectType<IsLowercase<`${number}${Capitalize<string>}`>>(false);
+
+expectType<IsLowercase<`${string}${Lowercase<string>}`>>({} as boolean);
+expectType<IsLowercase<`${string}${string}`>>({} as boolean);
+expectType<IsLowercase<`${Lowercase<string>}${Lowercase<string>}${string}`>>({} as boolean);
+expectType<IsLowercase<`${string}${Uncapitalize<string>}`>>({} as boolean);
+expectType<IsLowercase<`${number}/${number}`>>({} as boolean);
+expectType<IsLowercase<`${string}${number}`>>({} as boolean);
+
+// Unions
+expectType<IsLowercase<'abc' | 'xyz'>>(true); // Both `true`
+expectType<IsLowercase<'abC' | 'xYz'>>(false); // Both `false`
+expectType<IsLowercase<'abc' | 'Abc'>>({} as boolean); // One `true`, one `false`
+expectType<IsLowercase<'abc' | `${Uncapitalize<string>}end`>>({} as boolean); // One `true`, one `boolean`
+expectType<IsLowercase<'xYz' | `abc${string}`>>({} as boolean); // One `false`, one `boolean`
diff --git a/test-d/is-uppercase.ts b/test-d/is-uppercase.ts
new file mode 100644
index 00000000..2ce91a71
--- /dev/null
+++ b/test-d/is-uppercase.ts
@@ -0,0 +1,56 @@
+import {expectType} from 'tsd';
+import type {IsUppercase} from '../index.d.ts';
+
+// Literals
+expectType<IsUppercase<'ABC'>>(true);
+expectType<IsUppercase<''>>(true);
+expectType<IsUppercase<'123'>>(true);
+expectType<IsUppercase<'!@#'>>(true);
+
+expectType<IsUppercase<'Abc'>>(false);
+expectType<IsUppercase<'abc'>>(false);
+
+// Non-literals
+expectType<IsUppercase<Uppercase<string>>>(true);
+expectType<IsUppercase<Uppercase<`on${string}`>>>(true);
+
+expectType<IsUppercase<Lowercase<string>>>(false);
+expectType<IsUppercase<Uncapitalize<`ON${string}`>>>(false);
+expectType<IsUppercase<`ON${Uncapitalize<string>}`>>(false);
+expectType<IsUppercase<Uncapitalize<`${string}END`>>>(false);
+
+expectType<IsUppercase<string>>({} as boolean);
+expectType<IsUppercase<`ON${string}`>>({} as boolean);
+expectType<IsUppercase<Capitalize<string>>>({} as boolean);
+expectType<IsUppercase<Capitalize<`ON${string}`>>>({} as boolean);
+expectType<IsUppercase<`ON${Capitalize<string>}`>>({} as boolean);
+expectType<IsUppercase<`${number}`>>({} as boolean);
+
+expectType<IsUppercase<`sTART${string}`>>(false);
+expectType<IsUppercase<`${string}eND`>>(false);
+expectType<IsUppercase<`${string}mID${string}`>>(false);
+
+// Complex non-literals
+expectType<IsUppercase<`${Uppercase<string>}${Uppercase<string>}`>>(true);
+
+expectType<IsUppercase<`${Lowercase<string>}${Uppercase<string>}`>>(false);
+expectType<IsUppercase<`${Uppercase<string>}${Lowercase<string>}`>>(false);
+expectType<IsUppercase<`${Uppercase<string>}${Lowercase<string>}${Uppercase<string>}`>>(false);
+expectType<IsUppercase<`${Uncapitalize<string>}${Uppercase<string>}`>>(false);
+expectType<IsUppercase<`${Uppercase<string>}${Uncapitalize<string>}`>>(false);
+expectType<IsUppercase<`${string}${Uncapitalize<string>}`>>(false);
+expectType<IsUppercase<`${number}${Uncapitalize<string>}`>>(false);
+
+expectType<IsUppercase<`${string}${Uppercase<string>}`>>({} as boolean);
+expectType<IsUppercase<`${string}${string}`>>({} as boolean);
+expectType<IsUppercase<`${Uppercase<string>}${Uppercase<string>}${string}`>>({} as boolean);
+expectType<IsUppercase<`${string}${Capitalize<string>}`>>({} as boolean);
+expectType<IsUppercase<`${number}/${number}`>>({} as boolean);
+expectType<IsUppercase<`${string}${number}`>>({} as boolean);
+
+// Unions
+expectType<IsUppercase<'ABC' | 'XYZ'>>(true); // Both `true`
+expectType<IsUppercase<'ABc' | 'XyZ'>>(false); // Both `false`
+expectType<IsUppercase<'ABC' | 'aBC'>>({} as boolean); // One `true`, one `false`
+expectType<IsUppercase<'ABC' | `${Capitalize<string>}END`>>({} as boolean); // One `true`, one `boolean`
+expectType<IsUppercase<'XyZ' | `ABC${string}`>>({} as boolean); // One `false`, one `boolean`