diff --git a/graph/graph.go b/graph/graph.go
new file mode 100644
index 0000000..b225c92
--- /dev/null
+++ b/graph/graph.go
@@ -0,0 +1,156 @@
+// Package graph implements an immutable unidirectional graph.
+package graph
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"fmt"
+)
+
+// Value wraps a go value in a way such that it will be uniquely identified
+// within any Graph and between Graphs. Use NewValue to create a Value instance.
+// You can create an instance manually as long as ID is globally unique.
+type Value struct {
+	ID string
+	V  interface{}
+}
+
+// Void is the absence of any value.
+var Void Value
+
+// NewValue returns a Value instance wrapping any go value. The Value returned
+// will be independent of the passed in go value. So if the same go value is
+// passed in twice then the two returned Value instances will be treated as
+// being different values by Graph.
+func NewValue(V interface{}) Value {
+	b := make([]byte, 8)
+	if _, err := rand.Read(b); err != nil {
+		panic(err)
+	}
+	return Value{
+		ID: hex.EncodeToString(b),
+		V:  V,
+	}
+}
+
+// Edge is a directional edge connecting two values in a Graph, the Tail and the
+// Head. An Edge may also contain a value of its own.
+type Edge struct {
+	Tail, Val, Head Value
+}
+
+func (e Edge) id() string {
+	return fmt.Sprintf("%q-%q->%q", e.Tail, e.Val, e.Head)
+}
+
+// Graph implements an immutable, unidirectional graph which can hold generic
+// values. All methods are thread-safe as they don't modify the Graph in any
+// way.
+//
+// The Graph's zero value is the initial empty graph.
+//
+// The Graph does not keep track of Edge ordering. Assume that all slices of
+// Edges are in random order.
+type Graph struct {
+	m map[string]Edge
+}
+
+func (g Graph) cp() Graph {
+	g2 := Graph{
+		m: make(map[string]Edge, len(g.m)),
+	}
+	for id, e := range g.m {
+		g2.m[id] = e
+	}
+	return g2
+}
+
+// AddEdge returns a new Graph instance with the given Edge added to it. If the
+// original Graph already had that Edge this returns the original Graph.
+func (g Graph) AddEdge(e Edge) Graph {
+	id := e.id()
+	if _, ok := g.m[id]; ok {
+		return g
+	}
+
+	g2 := g.cp()
+	g2.m[id] = e
+	return g2
+}
+
+// DelEdge returns a new Graph instance without the given Edge in it. If the
+// original Graph didn't have that Edge  this returns the original Graph.
+func (g Graph) DelEdge(e Edge) Graph {
+	id := e.id()
+	if _, ok := g.m[id]; !ok {
+		return g
+	}
+
+	g2 := g.cp()
+	delete(g2.m, id)
+	return g2
+}
+
+// Values returns all Values which have incoming or outgoing Edges in the Graph.
+func (g Graph) Values() []Value {
+	values := make([]Value, 0, len(g.m))
+	found := map[string]bool{}
+	tryAdd := func(v Value) {
+		if ok := found[v.ID]; !ok {
+			values = append(values, v)
+			found[v.ID] = true
+		}
+	}
+
+	for _, e := range g.m {
+		tryAdd(e.Head)
+		tryAdd(e.Tail)
+	}
+	return values
+}
+
+// Edges returns all Edges which are part of the Graph
+func (g Graph) Edges() []Edge {
+	edges := make([]Edge, 0, len(g.m))
+	for _, e := range g.m {
+		edges = append(edges, e)
+	}
+	return edges
+}
+
+// ValueEdges returns all input (e.Head==v) and output (e.Tail==v) Edges
+// for the given Value in the Graph.
+func (g Graph) ValueEdges(v Value) ([]Edge, []Edge) {
+	var in, out []Edge
+	for _, e := range g.m {
+		if e.Tail.ID == v.ID {
+			out = append(out, e)
+		}
+		if e.Head.ID == v.ID {
+			in = append(in, e)
+		}
+	}
+	return in, out
+}
+
+// Traverse is used to traverse the Graph until a stopping point is reached.
+// Traversal starts with the cursor at the given start value. Each hop is
+// performed by passing the cursor value along with its input and output Edges
+// into the next function. The cursor moves to the returned Value and next is
+// called again, and so on.
+//
+// If the boolean returned from the next function is false traversal stops and
+// this method returns.
+//
+// If start has no Edges in the Graph, or a Value returned from next doesn't,
+// this will still call next, but the in/out params will both be empty.
+func (g Graph) Traverse(start Value, next func(v Value, in, out []Edge) (Value, bool)) {
+	curr := start
+	var ok bool
+	for {
+		in, out := g.ValueEdges(curr)
+		if curr, ok = next(curr, in, out); !ok {
+			return
+		}
+	}
+}
diff --git a/graph/graph_test.go b/graph/graph_test.go
new file mode 100644
index 0000000..111c871
--- /dev/null
+++ b/graph/graph_test.go
@@ -0,0 +1,135 @@
+package graph
+
+import (
+	. "testing"
+	"time"
+
+	"github.com/mediocregopher/mediocre-go-lib/mrand"
+	"github.com/mediocregopher/mediocre-go-lib/mtest/massert"
+	"github.com/mediocregopher/mediocre-go-lib/mtest/mchk"
+)
+
+func TestGraph(t *T) {
+	type state struct {
+		Graph
+
+		m map[string]Edge
+	}
+
+	type params struct {
+		add Edge
+		del Edge
+	}
+
+	strV := func(s string) Value {
+		return Value{ID: s, V: s}
+	}
+
+	chk := mchk.Checker{
+		Init: func() mchk.State {
+			return state{
+				m: map[string]Edge{},
+			}
+		},
+		Next: func(ss mchk.State) mchk.Action {
+			s := ss.(state)
+			var p params
+			if i := mrand.Intn(10); i == 0 {
+				// add edge which is already there
+				for _, e := range s.m {
+					p.add = e
+					break
+				}
+			} else if i == 1 {
+				// delete edge which isn't there
+				p.del = Edge{Tail: strV("z"), Val: strV("z"), Head: strV("z")}
+			} else if i <= 5 {
+				// add probably new edge
+				p.add = Edge{
+					Tail: strV(mrand.Hex(1)),
+					Val:  strV(mrand.Hex(1)),
+					Head: strV(mrand.Hex(1)),
+				}
+			} else {
+				// probably del edge
+				p.del = Edge{
+					Tail: strV(mrand.Hex(1)),
+					Val:  strV(mrand.Hex(1)),
+					Head: strV(mrand.Hex(1)),
+				}
+			}
+			return mchk.Action{Params: p}
+		},
+		Apply: func(ss mchk.State, a mchk.Action) (mchk.State, error) {
+			s, p := ss.(state), a.Params.(params)
+			if p.add != (Edge{}) {
+				s.Graph = s.Graph.AddEdge(p.add)
+				s.m[p.add.id()] = p.add
+			} else {
+				s.Graph = s.Graph.DelEdge(p.del)
+				delete(s.m, p.del.id())
+			}
+
+			{ // test Values and Edges methods
+				vals := s.Graph.Values()
+				edges := s.Graph.Edges()
+				var aa []massert.Assertion
+				found := map[string]bool{}
+				tryAssert := func(v Value) {
+					if ok := found[v.ID]; !ok {
+						found[v.ID] = true
+						aa = append(aa, massert.Has(vals, v))
+					}
+				}
+				for _, e := range s.m {
+					aa = append(aa, massert.Has(edges, e))
+					tryAssert(e.Head)
+					tryAssert(e.Tail)
+				}
+				aa = append(aa, massert.Len(vals, len(found)))
+				aa = append(aa, massert.Len(edges, len(s.m)))
+				if err := massert.All(aa...).Assert(); err != nil {
+					return nil, err
+				}
+			}
+
+			{ // test ValueEdges
+				for _, val := range s.Graph.Values() {
+					in, out := s.Graph.ValueEdges(val)
+					var expIn, expOut []Edge
+					for _, e := range s.m {
+						if e.Tail.ID == val.ID {
+							expOut = append(expOut, e)
+						}
+						if e.Head.ID == val.ID {
+							expIn = append(expIn, e)
+						}
+					}
+					if err := massert.Comment(massert.All(
+						massert.Subset(expIn, in),
+						massert.Len(in, len(expIn)),
+						massert.Subset(expOut, out),
+						massert.Len(out, len(expOut)),
+					), "val:%q", val.V).Assert(); err != nil {
+						return nil, err
+					}
+				}
+			}
+
+			return s, nil
+		},
+		MaxLength: 10,
+	}
+
+	err := chk.RunCase(
+		params{add: Edge{Tail: strV("4"), Val: strV("d"), Head: strV("4")}},
+		params{del: Edge{Tail: strV("4"), Val: strV("d"), Head: strV("4")}},
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err := chk.RunFor(5 * time.Second); err != nil {
+		t.Fatal(err)
+	}
+}