diff --git a/set.go b/set.go
new file mode 100644
index 0000000..f56a275
--- /dev/null
+++ b/set.go
@@ -0,0 +1,66 @@
+package multihash
+
+// Set is a set of Multihashes, holding one copy per Multihash.
+type Set struct {
+	set map[string]struct{}
+}
+
+// NewSet creates a new set correctly initialized.
+func NewSet() *Set {
+	return &Set{
+		set: make(map[string]struct{}),
+	}
+}
+
+// Add adds a new multihash to the set.
+func (s *Set) Add(m Multihash) {
+	s.set[string(m)] = struct{}{}
+}
+
+// Len returns the number of elements in the set.
+func (s *Set) Len() int {
+	return len(s.set)
+}
+
+// Has returns true if the element is in the set.
+func (s *Set) Has(m Multihash) bool {
+	_, ok := s.set[string(m)]
+	return ok
+}
+
+// Visit adds a multihash only if it is not in the set already.  Returns true
+// if the multihash was added (was not in the set before).
+func (s *Set) Visit(m Multihash) bool {
+	_, ok := s.set[string(m)]
+	if !ok {
+		s.set[string(m)] = struct{}{}
+		return true
+	}
+	return false
+}
+
+// ForEach runs f(m) with each multihash in the set. If returns immediately if
+// f(m) returns an error.
+func (s *Set) ForEach(f func(m Multihash) error) error {
+	for elem := range s.set {
+		mh := Multihash(elem)
+		if err := f(mh); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// Remove removes an element from the set.
+func (s *Set) Remove(m Multihash) {
+	delete(s.set, string(m))
+}
+
+// All returns a slice with all the elements in the set.
+func (s *Set) All() []Multihash {
+	out := make([]Multihash, 0, len(s.set))
+	for m := range s.set {
+		out = append(out, Multihash(m))
+	}
+	return out
+}
diff --git a/set_test.go b/set_test.go
new file mode 100644
index 0000000..2d3f607
--- /dev/null
+++ b/set_test.go
@@ -0,0 +1,86 @@
+package multihash
+
+import (
+	"crypto/rand"
+	"errors"
+	"testing"
+)
+
+func makeRandomMultihash(t *testing.T) Multihash {
+	t.Helper()
+
+	p := make([]byte, 256)
+	_, err := rand.Read(p)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	m, err := Sum(p, SHA3, 4)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return m
+}
+
+func TestSet(t *testing.T) {
+	mhSet := NewSet()
+
+	total := 10
+	for i := 0; i < total; i++ {
+		mhSet.Add(makeRandomMultihash(t))
+	}
+
+	m0 := makeRandomMultihash(t)
+
+	if mhSet.Len() != total {
+		t.Error("bad length")
+	}
+
+	if mhSet.Has(m0) {
+		t.Error("m0 should not be in set")
+	}
+
+	mhSet.Add(m0)
+
+	if !mhSet.Has(m0) {
+		t.Error("m0 should be in set")
+	}
+
+	i := 0
+	f := func(m Multihash) error {
+		i++
+		if i == 3 {
+			return errors.New("3")
+		}
+		return nil
+	}
+
+	mhSet.ForEach(f)
+	if i != 3 {
+		t.Error("forEach should have run 3 times")
+	}
+
+	mhSet.Remove(m0)
+
+	if mhSet.Len() != total {
+		t.Error("an element should have been removed")
+	}
+
+	if mhSet.Has(m0) {
+		t.Error("m0 should not be in set")
+	}
+
+	if !mhSet.Visit(m0) {
+		t.Error("Visit() should return true when new element added")
+	}
+
+	all := mhSet.All()
+	if len(all) != mhSet.Len() {
+		t.Error("All() should return all")
+	}
+	for _, mh := range all {
+		if !mhSet.Has(mh) {
+			t.Error("element in All() not in set")
+		}
+	}
+}