diff --git a/mtest/massert/massert.go b/mtest/massert/massert.go
index 7b94619..34549fc 100644
--- a/mtest/massert/massert.go
+++ b/mtest/massert/massert.go
@@ -8,6 +8,7 @@ import (
 	"path/filepath"
 	"reflect"
 	"runtime"
+	"strconv"
 	"strings"
 	"testing"
 	"text/tabwriter"
@@ -393,3 +394,18 @@ func HasKey(set, elem interface{}) Assertion {
 		return errors.New("value not a key in the map")
 	}, toStr(set)+" has key "+toStr(elem), 0)
 }
+
+// Len asserts that the given set has the given number of elements in it. The
+// set may be an array, a slice, or a map. A nil value'd set is considered to be
+// a length of zero.
+func Len(set interface{}, length int) Assertion {
+	return newAssertion(func() error {
+		setVV, err := toSet(set, false)
+		if err != nil {
+			return err
+		} else if len(setVV) != length {
+			return errors.New("set not correct length")
+		}
+		return nil
+	}, toStr(set)+" has length "+strconv.Itoa(length), 0)
+}
diff --git a/mtest/massert/massert_test.go b/mtest/massert/massert_test.go
index 703b62d..4ec2f67 100644
--- a/mtest/massert/massert_test.go
+++ b/mtest/massert/massert_test.go
@@ -172,3 +172,27 @@ func TestHasKey(t *T) {
 		HasKey(map[int]int{2: 2}, 1),
 	))
 }
+
+func TestLen(t *T) {
+	Fatal(t, All(
+		Len([]int(nil), 0),
+		Len([]int{}, 0),
+		Len([]int{1}, 1),
+		Len([]int{1, 2}, 2),
+		Len(map[int]int(nil), 0),
+		Len(map[int]int{}, 0),
+		Len(map[int]int{1: 1}, 1),
+		Len(map[int]int{1: 1, 2: 2}, 2),
+	))
+
+	Fatal(t, None(
+		Len([]int(nil), 1),
+		Len([]int{}, 1),
+		Len([]int{1}, 0),
+		Len([]int{1}, 2),
+		Len([]int{1, 2}, 1),
+		Len([]int{1, 2}, 3),
+		Len(map[int]int(nil), 1),
+		Len(map[int]int{}, 1),
+	))
+}