diff --git a/2023/benches/days.rs b/2023/benches/days.rs
index d079062..8662026 100644
--- a/2023/benches/days.rs
+++ b/2023/benches/days.rs
@@ -9,7 +9,7 @@ use criterion::Criterion;
 use aoc_2023::get_implementation;
 
 /// Number of days we have an implementation to benchmark
-const DAYS_IMPLEMENTED: u8 = 2;
+const DAYS_IMPLEMENTED: u8 = 3;
 
 fn read_input(day: u8) -> std::io::Result<Vec<u8>> {
     let input_path = format!("inputs/{day:02}.txt");
diff --git a/2023/src/day03.rs b/2023/src/day03.rs
index 7c1760f..b531ef8 100644
--- a/2023/src/day03.rs
+++ b/2023/src/day03.rs
@@ -1,7 +1,184 @@
-pub fn part1(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+use std::collections::HashMap;
+use std::ops::Index;
+
+use anyhow::Context;
+
+struct Grid<'a> {
+    width: usize,
+    data: &'a [u8],
 }
 
-pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+impl<'a> Grid<'a> {
+    pub fn new(data: &'a [u8]) -> anyhow::Result<Self> {
+        let width = 1 + data
+            .iter()
+            .position(|&c| c == b'\n')
+            .context("Failed to find end of line in grid")?;
+
+        anyhow::ensure!(
+            data.len() % width == 0,
+            "Grid should divide equally into rows"
+        );
+
+        Ok(Self { width, data })
+    }
+
+    pub fn height(&self) -> usize {
+        self.data.len() / self.width
+    }
+
+    pub fn width(&self) -> usize {
+        self.width - 1
+    }
+
+    pub fn rows(&self) -> impl Iterator<Item = &'a [u8]> {
+        let width = self.width();
+        self.data
+            .chunks_exact(self.width)
+            .map(move |row| &row[..width])
+    }
+}
+
+impl<'a> Index<usize> for Grid<'a> {
+    type Output = [u8];
+
+    fn index(&self, y: usize) -> &Self::Output {
+        let offset = y * self.width;
+        &self.data[offset..(offset + self.width())]
+    }
+}
+
+fn is_surrounded(grid: &Grid<'_>, y: usize, start: usize, last: usize) -> bool {
+    fn is_symbol(c: u8) -> bool {
+        match c {
+            b'0'..=b'9' => false,
+            b'.' => false,
+            b'\n' => false,
+            _ => true,
+        }
+    }
+    let x_min = start.saturating_sub(1);
+    let x_max = Ord::min(grid.width(), last + 2);
+
+    if y > 0 {
+        for nx in x_min..x_max {
+            if is_symbol(grid[y - 1][nx]) {
+                return true;
+            }
+        }
+    }
+
+    if y + 1 < grid.height() {
+        for nx in x_min..x_max {
+            if is_symbol(grid[y + 1][nx]) {
+                return true;
+            }
+        }
+    }
+
+    is_symbol(grid[y][x_min]) || is_symbol(grid[y][x_max - 1])
+}
+
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    let grid = Grid::new(input)?;
+    let mut sum = 0;
+
+    for (y, row) in grid.rows().enumerate() {
+        let mut it = row.iter().enumerate();
+        while let Some((start, &c)) = it.find(|&(_, c)| c.is_ascii_digit()) {
+            let mut end = start;
+            let mut num = u32::from(c - b'0');
+
+            for (x, &c) in it.by_ref().take_while(|&(_, c)| c.is_ascii_digit()) {
+                end = x;
+                num *= 10;
+                num += u32::from(c - b'0');
+            }
+
+            if is_surrounded(&grid, y, start, end) {
+                sum += num;
+            }
+        }
+    }
+
+    Ok(sum.to_string())
+}
+
+fn find_gear(grid: &Grid<'_>, y: usize, start: usize, end: usize) -> Option<(usize, usize)> {
+    let x_min = start.saturating_sub(1);
+    let x_max = Ord::min(grid.width(), end + 2);
+
+    if y > 0 {
+        for nx in x_min..x_max {
+            if grid[y - 1][nx] == b'*' {
+                return Some((nx, y - 1));
+            }
+        }
+    }
+
+    if y + 1 < grid.height() {
+        for nx in x_min..x_max {
+            if grid[y + 1][nx] == b'*' {
+                return Some((nx, y + 1));
+            }
+        }
+    }
+
+    for nx in [x_min, x_max - 1] {
+        if grid[y][nx] == b'*' {
+            return Some((nx, y));
+        }
+    }
+
+    None
+}
+
+pub fn part2(input: &[u8]) -> anyhow::Result<String> {
+    let grid = Grid::new(input)?;
+    let mut gears: HashMap<(usize, usize), (u32, u32)> = HashMap::new();
+
+    for (y, row) in grid.rows().enumerate() {
+        let mut it = row.iter().enumerate();
+        while let Some((start, &c)) = it.find(|&(_, c)| c.is_ascii_digit()) {
+            let mut end = start;
+            let mut num = u32::from(c - b'0');
+
+            for (x, &c) in it.by_ref().take_while(|&(_, c)| c.is_ascii_digit()) {
+                end = x;
+                num *= 10;
+                num += u32::from(c - b'0');
+            }
+
+            // Assumption: there is only one gear per number. This turns out to be true.
+            if let Some((gear_x, gear_y)) = find_gear(&grid, y, start, end) {
+                let entry = gears.entry((gear_x, gear_y)).or_insert((0, 1));
+                entry.0 += 1;
+                entry.1 *= num;
+            }
+        }
+    }
+
+    let sum: u32 = gears
+        .into_values()
+        .filter_map(|(count, ratio)| if count == 2 { Some(ratio) } else { None })
+        .sum();
+
+    Ok(sum.to_string())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/03.txt");
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!(part1(SAMPLE).unwrap(), "4361");
+    }
+
+    #[test]
+    fn sample_part2() {
+        assert_eq!(part2(SAMPLE).unwrap(), "467835");
+    }
 }