diff --git a/longest_collatz_sequence/collatz_test.go b/longest_collatz_sequence/collatz_test.go
new file mode 100644
index 0000000..5b9fb30
--- /dev/null
+++ b/longest_collatz_sequence/collatz_test.go
@@ -0,0 +1,79 @@
+package main
+
+import "testing"
+
+func TestCollatzNext(t *testing.T) {
+	tt := []struct {
+		name  string
+		input int
+		want  int
+	}{
+		{
+			"1",
+			1,
+			1,
+		},
+		{
+			"2",
+			2,
+			1,
+		},
+		{
+			"3",
+			3,
+			10,
+		},
+		{
+			"13",
+			13,
+			40,
+		},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			got := CollatzNext(tc.input)
+			if got != tc.want {
+				t.Errorf("CollatzNext(%d)=%d, want %d", tc.input, got, tc.want)
+			}
+		})
+	}
+}
+
+func TestIsEven(t *testing.T) {
+	tt := []struct {
+		name  string
+		input int
+		want  bool
+	}{
+		{
+			"1",
+			1,
+			false,
+		},
+		{
+			"2",
+			2,
+			true,
+		},
+		{
+			"99",
+			99,
+			false,
+		},
+		{
+			"100",
+			100,
+			true,
+		},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			got := IsEven(tc.input)
+			if got != tc.want {
+				t.Errorf("IsEven(%d)=%v, want %v", tc.input, got, tc.want)
+			}
+		})
+	}
+}
diff --git a/longest_collatz_sequence/main.go b/longest_collatz_sequence/main.go
new file mode 100644
index 0000000..144a5a5
--- /dev/null
+++ b/longest_collatz_sequence/main.go
@@ -0,0 +1,41 @@
+package main
+
+import "fmt"
+
+func main() {
+	num := 0
+	ml := 0
+	for i := 1000000; i > 1; i-- {
+		n := i
+		count := 1
+		for {
+			n = CollatzNext(n)
+			if n == 1 {
+				break
+			}
+			count++
+		}
+		if count > ml {
+			ml = count
+			num = i
+		}
+	}
+	fmt.Println("largest collatz is", num, "len:", ml)
+}
+
+func CollatzNext(n int) int {
+	if n == 1 {
+		return 1
+	} else if IsEven(n) {
+		return n / 2
+	} else {
+		return 3*n + 1
+	}
+}
+
+func IsEven(n int) bool {
+	if n > 1 && n%2 == 0 {
+		return true
+	}
+	return false
+}