diff --git a/src/array/sum.test.ts b/src/array/sum.test.ts
index ea43b61..c6d1815 100644
--- a/src/array/sum.test.ts
+++ b/src/array/sum.test.ts
@@ -54,6 +54,15 @@ describe("sum", () => {
     // Then we get back the sum of the numbers
     expect(result).toBe(18);
   });
+
+  it("works for bigints", () => {
+    // Given an array of bigints and undefined values
+    const a = [5n, undefined, 6n, 7n];
+    // When we call sum
+    const result = a.sum();
+    // Then we get back the sum of the bigints as a bigint
+    expect(result).toBe(18n);
+  });
 });
 
 describe("maybeSum", () => {
@@ -110,4 +119,13 @@ describe("maybeSum", () => {
     // Then we get back the maybeSum of the numbers
     expect(result).toBe(18);
   });
+
+  it("works for bigints", () => {
+    // Given an array of bigints and undefined values
+    const a = [5n, undefined, 6n, 7n];
+    // When we call maybeSum
+    const result = a.maybeSum();
+    // Then we get back the sum of the bigints as a bigint
+    expect(result).toBe(18n);
+  });
 });
diff --git a/src/array/sum.ts b/src/array/sum.ts
index b856b7e..6cb2a4e 100644
--- a/src/array/sum.ts
+++ b/src/array/sum.ts
@@ -6,6 +6,9 @@ declare global {
     sum(this: (number | undefined)[]): number;
     sum(f: CallbackFn<T, number | undefined>): number;
     sum(f: CallbackFn<T, Promise<number | undefined>>): Promise<number>;
+    sum(this: (bigint | undefined)[]): bigint;
+    sum(f: CallbackFn<T, bigint | undefined>): bigint;
+    sum(f: CallbackFn<T, Promise<bigint | undefined>>): Promise<bigint>;
     /**
      * Sums numbers but will return `undefined` if either a) there are no numbers or b) they are all
      * `null` or `undefined`.
@@ -17,12 +20,18 @@ declare global {
     maybeSum(this: (number | undefined)[]): number | undefined;
     maybeSum(f: CallbackFn<T, number | undefined>): number | undefined;
     maybeSum(f: CallbackFn<T, Promise<number | undefined>>): Promise<number | undefined>;
+    maybeSum(this: (bigint | undefined)[]): bigint | undefined;
+    maybeSum(f: CallbackFn<T, bigint | undefined>): bigint | undefined;
+    maybeSum(f: CallbackFn<T, Promise<bigint | undefined>>): Promise<bigint | undefined>;
   }
 
   interface ReadonlyArray<T> {
-    sum(this: (number | undefined)[]): number;
+    sum(this: readonly (number | undefined)[]): number;
     sum(f: CallbackFnRO<T, number | undefined>): number;
     sum(f: CallbackFnRO<T, Promise<number | undefined>>): Promise<number>;
+    sum(this: (bigint | undefined)[]): bigint;
+    sum(f: CallbackFn<T, bigint | undefined>): bigint;
+    sum(f: CallbackFn<T, Promise<bigint | undefined>>): Promise<bigint>;
     /**
      * Sums numbers but will return `undefined` if either a) there are no numbers or b) they are all
      * `null` or `undefined`.
@@ -31,23 +40,36 @@ declare global {
      * i.e. not being able to tell "is this zero b/c someone typed zero in?" vs. "is this zero b/c it
      * just doesn't have a value yet"?
      */
-    maybeSum(this: (number | undefined)[]): number | undefined;
+    maybeSum(this: readonly (number | undefined)[]): number | undefined;
     maybeSum(f: CallbackFnRO<T, number | undefined>): number | undefined;
     maybeSum(f: CallbackFnRO<T, Promise<number | undefined>>): Promise<number | undefined>;
+    maybeSum(this: readonly (bigint | undefined)[]): bigint | undefined;
+    maybeSum(f: CallbackFn<T, bigint | undefined>): bigint | undefined;
+    maybeSum(f: CallbackFn<T, Promise<bigint | undefined>>): Promise<bigint | undefined>;
   }
 }
 
 function sum<T, R extends number | undefined>(this: R[]): number;
 function sum<T, R extends number | undefined>(f: CallbackFn<T, R>): number;
 function sum<T, R extends number | undefined>(f: CallbackFn<T, Promise<R>>): Promise<number>;
-function sum<T, R extends number | undefined>(
+function sum<T, R extends bigint | undefined>(this: R[]): bigint;
+function sum<T, R extends bigint | undefined>(f: CallbackFn<T, R>): bigint;
+function sum<T, R extends bigint | undefined>(f: CallbackFn<T, Promise<R>>): Promise<bigint>;
+function sum<T, R extends number | bigint | undefined>(
   this: T[] | R[],
   fn?: CallbackFn<T, MaybePromise<R>>,
-): MaybePromise<number> {
+): MaybePromise<number | bigint> {
   const promisesOrNumbers = fn ? (this.map(fn as any) as R[] | Promise<R>[]) : (this as R[]);
-  return maybePromiseAllThen(promisesOrNumbers, (numbers) =>
-    numbers.reduce((result: number, number) => result + (number ?? 0), 0),
-  );
+  return maybePromiseAllThen(promisesOrNumbers, (numbers) => {
+    let sum: bigint | number | undefined = numbers[0];
+    for (let i = 1; i < numbers.length; i++) {
+      const number = numbers[i];
+      if (number !== undefined) {
+        sum = (((sum ?? (typeof number === "bigint" ? 0n : 0)) as any) + number) as any;
+      }
+    }
+    return sum ?? 0;
+  });
 }
 
 Array.prototype.sum = sum;
@@ -55,16 +77,24 @@ Array.prototype.sum = sum;
 function maybeSum<T, R extends number | undefined>(this: R[]): R;
 function maybeSum<T, R extends number | undefined>(f: CallbackFn<T, R>): R;
 function maybeSum<T, R extends number | undefined>(f: CallbackFn<T, Promise<R>>): Promise<R>;
-function maybeSum<T, R extends number | undefined>(
+function maybeSum<T, R extends bigint | undefined>(this: R[]): R;
+function maybeSum<T, R extends bigint | undefined>(f: CallbackFn<T, R>): R;
+function maybeSum<T, R extends bigint | undefined>(f: CallbackFn<T, Promise<R>>): Promise<R>;
+function maybeSum<T, R extends number | bigint | undefined>(
   this: T[] | R[],
   fn?: CallbackFn<T, MaybePromise<R>>,
-): MaybePromise<number | undefined> {
+): MaybePromise<number | bigint | undefined> {
   const promisesOrNumbers = fn ? (this.map(fn as any) as R[] | Promise<R>[]) : (this as R[]);
-  return maybePromiseAllThen(promisesOrNumbers, (numbers) =>
-    numbers.every((n) => n === undefined)
-      ? undefined
-      : numbers.reduce((result: number, number) => result + (number ?? 0), 0),
-  );
+  return maybePromiseAllThen(promisesOrNumbers, (numbers) => {
+    let sum: bigint | number | undefined = numbers[0];
+    for (let i = 1; i < numbers.length; i++) {
+      const number = numbers[i];
+      if (number !== undefined) {
+        sum = (((sum ?? (typeof number === "bigint" ? 0n : 0)) as any) + number) as any;
+      }
+    }
+    return sum;
+  });
 }
 
 Array.prototype.maybeSum = maybeSum;