diff --git a/index.d.ts b/index.d.ts
index b00eea7da..f39c81ac6 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -158,6 +158,9 @@ export type {IsUppercase} from './source/is-uppercase.d.ts';
 export type {IsOptional} from './source/is-optional.d.ts';
 export type {IsNullable} from './source/is-nullable.d.ts';
 export type {TupleOf} from './source/tuple-of.d.ts';
+export type {IndexOf} from './source/index-of.d.ts';
+export type {LastIndexOf} from './source/last-index-of.d.ts';
+export type {IndicesOf} from './source/indices-of.d.ts';
 
 // Template literal types
 export type {CamelCase, CamelCaseOptions} from './source/camel-case.d.ts';
diff --git a/source/index-of.d.ts b/source/index-of.d.ts
new file mode 100644
index 000000000..5cf791389
--- /dev/null
+++ b/source/index-of.d.ts
@@ -0,0 +1,64 @@
+import type {GreaterThanOrEqual} from './greater-than-or-equal.d.ts';
+import type {ApplyDefaultOptions} from './internal/object.d.ts';
+import type {IsOptionalKeyOf} from './is-optional-key-of.d.ts';
+import type {ReverseSign} from './internal/numeric.d.ts';
+import type {UnknownArray} from './unknown-array.d.ts';
+import type {IsAnyOrNever} from './internal/type.d.ts';
+import type {LiteralUnion} from './literal-union.d.ts';
+import type {IsEqual} from './is-equal.d.ts';
+
+/* eslint-disable type-fest/require-exported-types */
+export type Inc<T extends number[]> = [...T, 1];
+export type Neg<T extends number[]> = ReverseSign<T['length']>;
+export type IndexOfOptions = {
+	ignoreOptionalModifier?: boolean;
+	fromIndex?: number;
+};
+/* eslint-enable type-fest/require-exported-types */
+
+type DefaultIndexOfOptions = {
+	ignoreOptionalModifier: false;
+	fromIndex: 0;
+};
+
+export type IndexOf<
+	Array_ extends UnknownArray, Item,
+	Options extends IndexOfOptions = {},
+> = IsAnyOrNever<Array_> extends true ? Array_
+	: _IndexOf<Array_, Item,
+		ApplyDefaultOptions<
+			IndexOfOptions,
+			DefaultIndexOfOptions,
+			Options
+		>
+	>;
+
+type _IndexOf<
+	Array_ extends UnknownArray, Item,
+	Options extends Required<IndexOfOptions>,
+	F_Index extends number[] = [],
+	B_Index extends number[] = [1],
+	T_Index extends number[] = never,
+> =
+	keyof Array_ & `${number}` extends never
+		// Backward search
+		? Array_ extends readonly [...infer Rest, infer Last]
+			? IsEqual<Last, Item> extends true
+				? _IndexOf<Rest, Item, Options, F_Index, Inc<B_Index>, B_Index>
+				: _IndexOf<Rest, Item, Options, F_Index, Inc<B_Index>, T_Index>
+			: Array_ extends readonly [] ? Neg<T_Index>
+				: IsEqual<Array_[number], Item> extends false ? Neg<T_Index>
+					: LiteralUnion<F_Index, number>
+		// Forward search
+		: Array_ extends readonly [(infer First)?, ...infer Rest]
+			? GreaterThanOrEqual<F_Index['length'], Options['fromIndex']> extends true
+				? IsEqual<First, Item> extends true ? F_Index['length'] | (
+					Options['ignoreOptionalModifier'] extends true ? never
+						: IsOptionalKeyOf<Array_, '0'> extends false ? never
+							: _IndexOf<Rest, Item, Options, Inc<F_Index>>)
+					: _IndexOf<Rest, Item, Options, Inc<F_Index>>
+				: _IndexOf<Rest, Item, Options, Inc<F_Index>>
+
+			: never;
+
+export {};
diff --git a/source/indices-of.d.ts b/source/indices-of.d.ts
new file mode 100644
index 000000000..49610f901
--- /dev/null
+++ b/source/indices-of.d.ts
@@ -0,0 +1,66 @@
+import type {ApplyDefaultOptions} from './internal/object.d.ts';
+import type {IsOptionalKeyOf} from './is-optional-key-of.d.ts';
+import type {UnknownArray} from './unknown-array.d.ts';
+import type {IsAnyOrNever} from './internal/type.d.ts';
+import type {Inc, Neg} from './index-of.d.ts';
+import type {IsEqual} from './is-equal.d.ts';
+
+type IndicesOfOptions = {
+	ignoreOptionalModifier?: boolean;
+	// FromIndex?: number;
+};
+
+type DefaultIndicesOfOptions = {
+	ignoreOptionalModifier: false;
+	// FromIndex: 0;
+};
+
+export type IndicesOf<
+	Array_ extends UnknownArray, Item,
+	Options extends IndicesOfOptions = {},
+> = IsAnyOrNever<Array_> extends true ? Array_
+	: _IndicesOf<Array_, Item,
+		ApplyDefaultOptions<
+			IndicesOfOptions,
+			DefaultIndicesOfOptions,
+			Options
+		>
+	>;
+
+type _IndicesOf<
+	Array_ extends UnknownArray, Item,
+	Options extends Required<IndicesOfOptions>,
+	F_Index extends number[] = [],
+	F_Indices extends Array<number | undefined> = [],
+	B_Index extends number[] = [1],
+	B_Indices extends Array<number | undefined> = [],
+	T_Index extends number = F_Index['length'],
+> =
+	keyof Array_ & `${number}` extends never
+
+		? Array_ extends readonly [...infer Rest, infer Last]
+			? IsEqual<Last, Item> extends true
+				? _IndicesOf<Rest, Item, Options, F_Index, F_Indices, Inc<B_Index>, [Neg<B_Index>, ...B_Indices]>
+				: _IndicesOf<Rest, Item, Options, F_Index, F_Indices, Inc<B_Index>, B_Indices>
+
+			: [
+				...F_Indices,
+				...Array_ extends readonly [] ? []
+					: IsEqual<Array_[number], Item> extends false ? []
+						: [Array_[number] | (number & {})],
+				...B_Indices,
+			]
+
+		: Array_ extends readonly [(infer First)?, ...infer Rest]
+			? IsEqual<First, Item> extends true
+				? _IndicesOf<Rest, Item, Options, Inc<F_Index>, [
+					...F_Indices,
+					...Options['ignoreOptionalModifier'] extends true ? [T_Index]
+						: IsOptionalKeyOf<Array_, '0'> extends false ? [T_Index]
+							: [T_Index?],
+				]>
+				: _IndicesOf<Rest, Item, Options, Inc<F_Index>, F_Indices>
+
+			: never;
+
+export {};
diff --git a/source/last-index-of.d.ts b/source/last-index-of.d.ts
new file mode 100644
index 000000000..7f7f9d668
--- /dev/null
+++ b/source/last-index-of.d.ts
@@ -0,0 +1,74 @@
+import type {GreaterThanOrEqual} from './greater-than-or-equal.d.ts';
+import type {ApplyDefaultOptions} from './internal/object.d.ts';
+import type {IsOptionalKeyOf} from './is-optional-key-of.d.ts';
+import type {Inc, Neg, IndexOfOptions} from './index-of.d.ts';
+import type {UnknownArray} from './unknown-array.d.ts';
+import type {IsAnyOrNever} from './internal/type.d.ts';
+import type {LiteralUnion} from './literal-union.d.ts';
+import type {GreaterThan} from './greater-than.d.ts';
+import type {IsEqual} from './is-equal.d.ts';
+
+type DefaultLastIndexOfOptions = {
+	ignoreOptionalModifier: false;
+	fromIndex: 0;
+};
+
+type IsLastOptional<T extends UnknownArray> =
+	T extends [...infer Rest, (infer _)?]
+		? [...Rest] extends T
+			? true
+			: false
+		: false;
+
+export type LastIndexOf<
+	Array_ extends UnknownArray, Item,
+	Options extends IndexOfOptions = {},
+> = IsAnyOrNever<Array_> extends true ? Array_
+	: _LastIndexOf<Array_, Item,
+		ApplyDefaultOptions<
+			IndexOfOptions,
+			DefaultLastIndexOfOptions,
+			Options
+		>
+	>;
+
+type _LastIndexOf<
+	Array_ extends UnknownArray, Item,
+	Options extends Required<IndexOfOptions>,
+	F_Index extends number[] = [],
+	B_Index extends number[] = [1],
+	T_Index extends number = never,
+> =
+	number extends Array_['length']
+		? keyof Array_ & `${number}` extends never
+
+			? Array_ extends readonly [...infer Rest, infer Last]
+				? GreaterThan<B_Index['length'], Options['fromIndex']> extends true
+					? IsEqual<Last, Item> extends true ? Neg<B_Index>
+						: _LastIndexOf<Rest, Item, Options, F_Index, Inc<B_Index>, T_Index>
+					: _LastIndexOf<Rest, Item, Options, F_Index, Inc<B_Index>, T_Index>
+
+				: Array_ extends readonly [] ? T_Index
+					: IsEqual<Array_[number], Item> extends false ? T_Index
+						: LiteralUnion<F_Index['length'], number>
+
+			: Array_ extends readonly [(infer First)?, ...infer Rest]
+				? IsEqual<First, Item> extends true
+					? _LastIndexOf<Rest, Item, Options, Inc<F_Index>, B_Index, F_Index['length'] | (
+						Options['ignoreOptionalModifier'] extends true ? never
+							: IsOptionalKeyOf<Array_, '0'> extends false ? never
+								: _LastIndexOf<Rest, Item, Options, Inc<F_Index>, B_Index, T_Index>)>
+					: _LastIndexOf<Rest, Item, Options, Inc<F_Index>, B_Index, T_Index>
+				: never
+
+		: Array_ extends readonly [...infer Rest, (infer Last)?]
+			? GreaterThanOrEqual<F_Index['length'], Options['fromIndex']> extends true
+				? IsEqual<Last, Item> extends true ? Required<Rest>['length'] | (
+					Options['ignoreOptionalModifier'] extends true ? never
+						: IsLastOptional<Array_> extends false ? never
+							: _LastIndexOf<Rest, Item, Options, Inc<F_Index>>)
+					: _LastIndexOf<Rest, Item, Options, Inc<F_Index>>
+				: _LastIndexOf<Rest, Item, Options, Inc<F_Index>>
+			: never;
+
+export {};