diff --git a/2015/day18/main.go b/2015/day18/main.go
new file mode 100644
index 0000000..2432466
--- /dev/null
+++ b/2015/day18/main.go
@@ -0,0 +1,78 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	ans := gameOfLight(util.ReadFile("./input.txt"), part)
+	fmt.Println("Output:", ans)
+}
+
+func gameOfLight(input string, part int) int {
+	var grid [][]string
+	for _, row := range strings.Split(input, "\n") {
+		grid = append(grid, strings.Split(row, ""))
+	}
+
+	for i := 0; i < 100; i++ {
+		grid = tick(grid)
+		if part == 2 {
+			grid[0][0] = "#"
+			grid[0][len(grid[0])-1] = "#"
+			grid[len(grid)-1][0] = "#"
+			grid[len(grid)-1][len(grid[0])-1] = "#"
+		}
+	}
+
+	var count int
+	for _, row := range grid {
+		for _, c := range row {
+			if c == "#" {
+				count++
+			}
+		}
+	}
+
+	return count
+}
+
+func tick(grid [][]string) [][]string {
+	var nextGrid [][]string
+	for r, row := range grid {
+		nextGrid = append(nextGrid, make([]string, len(grid[0])))
+		for c, cell := range row {
+			var neighbors int
+			for rDiff := -1; rDiff <= 1; rDiff++ {
+				for cDiff := -1; cDiff <= 1; cDiff++ {
+					if !(rDiff == 0 && cDiff == 0) {
+						nextRow := r + rDiff
+						nextCol := c + cDiff
+						if nextRow >= 0 && nextRow < len(grid) && nextCol >= 0 && nextCol < len(grid[0]) &&
+							grid[nextRow][nextCol] == "#" {
+							neighbors++
+						}
+					}
+				}
+			}
+			if cell == "#" && (neighbors == 2 || neighbors == 3) {
+				nextGrid[r][c] = "#"
+			} else if cell == "." && neighbors == 3 {
+				nextGrid[r][c] = "#"
+			} else {
+				nextGrid[r][c] = "."
+			}
+		}
+	}
+
+	return nextGrid
+}
diff --git a/2015/day18/main_test.go b/2015/day18/main_test.go
new file mode 100644
index 0000000..c3734c1
--- /dev/null
+++ b/2015/day18/main_test.go
@@ -0,0 +1,26 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func Test_gameOfLight(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		part  int
+		want  int
+	}{
+		{"part1 actual", util.ReadFile("input.txt"), 1, 768},
+		{"part2 actual", util.ReadFile("input.txt"), 2, 781},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := gameOfLight(tt.input, tt.part); got != tt.want {
+				t.Errorf("gameOfLight() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}