diff --git a/mod.test.ts b/mod.test.ts
index f1facf5..359168b 100644
--- a/mod.test.ts
+++ b/mod.test.ts
@@ -20,7 +20,9 @@ import {
   imapfilter,
   imaterialize,
   imax,
+  imaxBy,
   imin,
+  iminBy,
   ipartition,
   irange,
   irecur,
@@ -340,5 +342,11 @@ describe("Iterators", () => {
 
     expect(imax(IEMPTY)).toEqual(-Infinity);
     expect(imax(iarray([3, 8, 2, 4]))).toEqual(8);
+
+    expect(iminBy(IEMPTY, (x) => x)).toBeUndefined();
+    expect(iminBy(["aaa", "b", "cc"], (x) => x.length)).toEqual("b");
+
+    expect(imaxBy(IEMPTY, (x) => x)).toBeUndefined();
+    expect(imaxBy(["aaa", "b", "cc"], (x) => x.length)).toEqual("aaa");
   });
 });
diff --git a/mod.ts b/mod.ts
index c2c0eb8..5e3b0f4 100644
--- a/mod.ts
+++ b/mod.ts
@@ -516,3 +516,27 @@ export const imin = (iterable: Iterable<number>) =>
   ireduce(iterable, Math.min, Infinity);
 export const imax = (iterable: Iterable<number>) =>
   ireduce(iterable, Math.max, -Infinity);
+
+/**
+ * Min/max by key function
+ */
+export function iminBy<T>(
+  iterable: Iterable<T>,
+  key: (value: T) => number,
+): T | undefined {
+  return ireduce(
+    iterable,
+    (item1, item2) =>
+      (typeof item1 == "undefined" || typeof item2 == "undefined") ||
+        (key(item1) > key(item2))
+        ? item2
+        : item1,
+    undefined,
+  );
+}
+export function imaxBy<T>(
+  iterable: Iterable<T>,
+  key: (value: T) => number,
+): T | undefined {
+  return iminBy(iterable, (value) => -key(value));
+}