diff --git a/2021/day11/main.go b/2021/day11/main.go
new file mode 100644
index 0000000..937791c
--- /dev/null
+++ b/2021/day11/main.go
@@ -0,0 +1,134 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/fatih/color"
+	"github.com/jdkaplan/advent-of-code/aoc"
+)
+
+func main() {
+	lines := aoc.Input().ReadLines("day11.txt")
+	fmt.Println(part1(lines))
+	fmt.Println(part2(lines))
+}
+
+func part1(lines []string) (flashes int) {
+	g := NewGrid(lines)
+	for i := 0; i < 100; i++ {
+		count := g.Tick()
+		flashes += count
+	}
+	return
+}
+
+func part2(lines []string) int {
+	g := NewGrid(lines)
+	all := g.Rows * g.Cols
+	for i := 0; true; i++ {
+		if g.Tick() == all {
+			return i + 1
+		}
+	}
+	return 0
+}
+
+type RC struct{ r, c int }
+
+type Grid struct {
+	Rows int
+	Cols int
+
+	m map[RC]int
+}
+
+func NewGrid(lines []string) Grid {
+	g := Grid{
+		m:    make(map[RC]int),
+		Rows: len(lines),
+		Cols: len(lines[0]),
+	}
+	for r, row := range lines {
+		for c, col := range row {
+			g.m[RC{r, c}] = int(col - '0')
+		}
+	}
+	return g
+}
+
+var red = color.New(color.FgRed)
+
+func (g Grid) String() string {
+	var sb strings.Builder
+	for r := 0; r < g.Rows; r++ {
+		for c := 0; c < g.Cols; c++ {
+			if e := g.m[RC{r, c}]; e == 0 {
+				red.Fprintf(&sb, "%d", e)
+			} else {
+				fmt.Fprintf(&sb, "%d", e)
+			}
+		}
+		sb.WriteString("\n")
+	}
+	return sb.String()
+}
+
+func (g Grid) Equal(o Grid) bool {
+	for r := 0; r < g.Rows; r++ {
+		for c := 0; c < g.Cols; c++ {
+			rc := RC{r, c}
+			if g.m[rc] != o.m[rc] {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+func (g *Grid) Tick() (flashes int) {
+	var frontier []RC
+	flashed := make(map[RC]bool)
+	for r := 0; r < g.Rows; r++ {
+		for c := 0; c < g.Cols; c++ {
+			rc := RC{r, c}
+			g.m[rc]++
+			if g.m[rc] > 9 {
+				frontier = append(frontier, rc)
+			}
+		}
+	}
+
+	for len(frontier) > 0 {
+		rc := frontier[0]
+		frontier = frontier[1:]
+		if flashed[rc] {
+			continue
+		}
+		for _, n := range g.Neighbors(rc) {
+			g.m[n]++
+			if g.m[n] > 9 {
+				frontier = append(frontier, n)
+			}
+		}
+		flashed[rc] = true
+	}
+	for rc := range flashed {
+		g.m[rc] = 0
+	}
+	return len(flashed)
+}
+
+func (g Grid) Neighbors(rc RC) (ns []RC) {
+	r, c := rc.r, rc.c
+	for _, n := range []RC{
+		{r - 1, c - 1}, {r - 1, c}, {r - 1, c + 1},
+		{r, c - 1}, {r, c + 1},
+		{r + 1, c - 1}, {r + 1, c}, {r + 1, c + 1},
+	} {
+		if 0 <= n.r && n.r < g.Rows && 0 <= n.c && n.c < g.Cols {
+			ns = append(ns, n)
+		}
+	}
+	return
+}