diff --git a/2021/src/day1.rs b/2021/src/day1.rs
index f913a8f..08e0caa 100644
--- a/2021/src/day1.rs
+++ b/2021/src/day1.rs
@@ -40,6 +40,38 @@
 //! In this example, there are 7 measurements that are larger than the previous measurement.
 //!
 //! How many measurements are larger than the previous measurement?
+//! --- Part Two ---
+//! Considering every single measurement isn't as useful as you expected: there's just too much noise in the data.
+//!
+//! Instead, consider sums of a three-measurement sliding window. Again considering the above example:
+//!
+//! 199  A
+//! 200  A B
+//! 208  A B C
+//! 210    B C D
+//! 200  E   C D
+//! 207  E F   D
+//! 240  E F G
+//! 269    F G H
+//! 260      G H
+//! 263        H
+//! Start by comparing the first and second three-measurement windows. The measurements in the first window are marked A (199, 200, 208); their sum is 199 + 200 + 208 = 607. The second window is marked B (200, 208, 210); its sum is 618. The sum of measurements in the second window is larger than the sum of the first, so this first comparison increased.
+//!
+//! Your goal now is to count the number of times the sum of measurements in this sliding window increases from the previous sum. So, compare A with B, then compare B with C, then C with D, and so on. Stop when there aren't enough measurements left to create a new three-measurement sum.
+//!
+//! In the above example, the sum of each three-measurement window is as follows:
+//!
+//! A: 607 (N/A - no previous sum)
+//! B: 618 (increased)
+//! C: 618 (no change)
+//! D: 617 (decreased)
+//! E: 647 (increased)
+//! F: 716 (increased)
+//! G: 769 (increased)
+//! H: 792 (increased)
+//! In this example, there are 5 sums that are larger than the previous sum.
+//!
+//! Consider sums of a three-measurement sliding window. How many sums are larger than the previous sum?
 
 use aoc_runner_derive::{aoc, aoc_generator};
 
@@ -61,6 +93,14 @@ fn part1(depths: &[u32]) -> u32 {
         .sum()
 }
 
+#[aoc(day1, part2)]
+fn part2(depths: &[u32]) -> u32 {
+    let sums: Vec<u32> = depths.windows(3).map(|s| s.iter().sum()).collect();
+    sums.windows(2)
+        .map(|s| if s[0] < s[1] { 1 } else { 0 })
+        .sum()
+}
+
 #[test]
 fn test_part1() {
     assert_eq!(
@@ -79,3 +119,22 @@ fn test_part1() {
         7
     );
 }
+
+#[test]
+fn test_part2() {
+    assert_eq!(
+        part2(&parse(
+            r#"199
+200
+208
+210
+200
+207
+240
+269
+260
+263"#
+        )),
+        5
+    );
+}