diff --git a/README.md b/README.md
index 21334c0..7fe7d13 100644
--- a/README.md
+++ b/README.md
@@ -58,6 +58,8 @@ isum(i1); // 6
 icat(iarray(["a", "b", "c"])); // abc
 imin(i1); // 1
 imax(i1); // 3
+iminBy(i1, (x) => -x); // 3
+imaxBy(i1, (x) => -x); // 1
 
 // Transform iterables
 iloop(i1); // 1 2 3 1 2 3 ...
@@ -67,6 +69,7 @@ ifilter(i3, (x) => x % 4 == 0); // 0 -4 -8 -12 ...
 imapfilter(i1, (x) => x != 4 ? x * 2 : null); // 2 6
 ipartition(i3, (x) => x % 4 == 0); // [0 -4 -8 ..., -1 -2 -3 -5 ...]
 iunique(iarray([1, 4, 2, 4, 3, 1, 5])); // 1 4 2 3 5
+ienumerate(iarray(["a", "b", "c"])); // [0, "a"] [1, "b"] [2, "c"]
 
 // Combine iterables
 ichain(i1, i2); // 1 2 3 4
diff --git a/doc/use.md b/doc/use.md
index e4118bf..468b169 100644
--- a/doc/use.md
+++ b/doc/use.md
@@ -35,6 +35,7 @@ ifilter(i3, (x) => x % 4 == 0); // 0 -4 -8 -12 ...
 imapfilter(i1, (x) => x != 4 ? x * 2 : null); // 2 6
 ipartition(i3, (x) => x % 4 == 0); // [0 -4 -8 ..., -1 -2 -3 -5 ...]
 iunique(iarray([1, 4, 2, 4, 3, 1, 5])); // 1 4 2 3 5
+ienumerate(iarray(["a", "b", "c"])); // [0, "a"] [1, "b"] [2, "c"]
 
 // Combine iterables
 ichain(i1, i2); // 1 2 3 4
diff --git a/mod.test.ts b/mod.test.ts
index 359168b..04c7a16 100644
--- a/mod.test.ts
+++ b/mod.test.ts
@@ -8,6 +8,7 @@ import {
   ichainit,
   icombine,
   IEMPTY,
+  ienumerate,
   ifilter,
   ifilterclass,
   ifiltertype,
@@ -286,6 +287,15 @@ describe("Iterators", () => {
     ]);
   });
 
+  it("enumerate iterators", () => {
+    let iterator = ienumerate(iarray(["a", "b", "c"]));
+    checkit(iterator, [
+      [0, "a"],
+      [1, "b"],
+      [2, "c"],
+    ]);
+  });
+
   it("zips iterators", () => {
     checkit(izip(IEMPTY, IEMPTY), []);
     checkit(izip(iarray([1, 2, 3]), iarray(["a", "b"])), [[1, "a"], [
diff --git a/mod.ts b/mod.ts
index 5e3b0f4..5e89167 100644
--- a/mod.ts
+++ b/mod.ts
@@ -390,6 +390,24 @@ export function icombine<T1, T2>(
   return ichainit(imap(it1, (v1) => imap(it2, (v2): [T1, T2] => [v1, v2])));
 }
 
+/**
+ * Enumerate another iterable, adding the index before the value.
+ */
+export function ienumerate<T>(it: Iterable<T>): Iterable<[number, T]> {
+  return {
+    [Symbol.iterator]: function* () {
+      const iterator = it[Symbol.iterator]();
+      let state = iterator.next();
+      let idx = 0;
+      while (!state.done) {
+        yield [idx, state.value];
+        state = iterator.next();
+        idx += 1;
+      }
+    },
+  };
+}
+
 /**
  * Advance through two iterables at the same time, yielding item pairs
  *