diff --git a/advent-of-code-2021.csproj b/advent-of-code-2021.csproj
index 6a8a541..b585d03 100644
--- a/advent-of-code-2021.csproj
+++ b/advent-of-code-2021.csproj
@@ -48,6 +48,9 @@
 	  <None Update="inputs\10.txt">
 	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
 	  </None>
+	  <None Update="inputs\11.txt">
+	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+	  </None>
 	</ItemGroup>
 
 </Project>
diff --git a/inputs/11.txt b/inputs/11.txt
new file mode 100644
index 0000000..328ac4f
--- /dev/null
+++ b/inputs/11.txt
@@ -0,0 +1,10 @@
+3265255276
+1537412665
+7335746422
+6426325658
+3854434364
+8717377486
+4522286326
+6337772845
+8824387665
+6351586484
\ No newline at end of file
diff --git a/src/11.cs b/src/11.cs
new file mode 100644
index 0000000..c7cf47c
--- /dev/null
+++ b/src/11.cs
@@ -0,0 +1,142 @@
+namespace aoc2021;
+
+internal class Day11
+{
+    internal static void Go()
+    {
+        Logger.Log("Day 11");
+        Logger.Log("-----");
+
+        var lines = File.ReadAllLines("inputs/11.txt");
+        var grid = new byte[lines.Length, lines[0].Length];
+        for (int i = 0; i < lines.Length; i++)
+        {
+            var line = lines[i];
+            for (int j = 0; j < line.Length; j++)
+            {
+                var num = line[j];
+                grid[i, j] = (byte)char.GetNumericValue(num);
+            }
+        }
+
+        //var gridCopy = new byte[lines.Length, lines[0].Length];
+        //Buffer.BlockCopy(grid, 0, gridCopy, 0, grid.Length);
+
+        Part1(grid);
+        Part2(grid);
+
+        Logger.Log("");
+    }
+
+    private static IEnumerable<(int, int)> Adjacent(byte[,] grid, int i, int j)
+    {
+        if (i > 0)
+        {
+            if (j > 0)
+            {
+                yield return (i - 1, j - 1);
+            }
+            yield return (i - 1, j);
+            if (j < grid.GetLength(1) - 1)
+            {
+                yield return (i - 1, j + 1);
+            }
+        }
+        if (j > 0)
+        {
+            yield return (i, j - 1);
+        }
+        if (j < grid.GetLength(1) - 1)
+        {
+            yield return (i, j + 1);
+        }
+        if (i < grid.GetLength(1) - 1)
+        {
+            if (j > 0)
+            {
+                yield return (i + 1, j - 1);
+            }
+            yield return (i + 1, j);
+            if (j < grid.GetLength(1) - 1)
+            {
+                yield return (i + 1, j + 1);
+            }
+        }
+    }
+
+    private static void Flash(byte[,] grid, int i, int j, List<(int, int)> flashed)
+    {
+        flashed.Add((i, j));
+        foreach (var pt in Adjacent(grid, i, j))
+        {
+            grid[pt.Item1, pt.Item2]++;
+            if (grid[pt.Item1, pt.Item2] == 10)
+            {
+                Flash(grid, pt.Item1, pt.Item2, flashed);
+            }
+        }
+    }
+
+    private static void Part1(byte[,] grid)
+    {
+        using var t = new Timer();
+
+        long numFlashes = 0;
+        for (int step = 0; step < 100; step++)
+        {
+            var flashed = new List<(int, int)>();
+            for (int i = 0; i < grid.GetLength(0); i++)
+            {
+                for (int j = 0; j < grid.GetLength(1); j++)
+                {
+                    grid[i, j]++;
+                    if (grid[i, j] == 10)
+                    {
+                        Flash(grid, i, j, flashed);
+                    }
+                }
+            }
+
+            foreach (var pt in flashed)
+            {
+                grid[pt.Item1, pt.Item2] = 0;
+                numFlashes++;
+            }
+        }
+
+        Logger.Log($"part1: {numFlashes}");
+    }
+
+    private static void Part2(byte[,] grid)
+    {
+        using var t = new Timer();
+
+        int step = 101;
+        for (; ; step++)
+        {
+            var flashed = new List<(int, int)>();
+            for (int i = 0; i < grid.GetLength(0); i++)
+            {
+                for (int j = 0; j < grid.GetLength(1); j++)
+                {
+                    grid[i, j]++;
+                    if (grid[i, j] == 10)
+                    {
+                        Flash(grid, i, j, flashed);
+                    }
+                }
+            }
+
+            if (flashed.Count == grid.GetLength(0) * grid.GetLength(1))
+            {
+                break;
+            }
+            foreach (var pt in flashed)
+            {
+                grid[pt.Item1, pt.Item2] = 0;
+            }
+        }
+
+        Logger.Log($"part2: {step}");
+    }
+}
diff --git a/src/main.cs b/src/main.cs
index afd62e0..2915053 100644
--- a/src/main.cs
+++ b/src/main.cs
@@ -33,7 +33,11 @@ switch (arg)
         aoc2021.Day09.Go();
         break;
 
-    default:
+    case "10":
         aoc2021.Day10.Go();
         break;
+
+    default:
+        aoc2021.Day11.Go();
+        break;
 }