diff --git a/internal/diff/diff.go b/internal/diff/diff.go
new file mode 100644
index 0000000..e219cce
--- /dev/null
+++ b/internal/diff/diff.go
@@ -0,0 +1,307 @@
+// Copyright 2019 CUE Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package diff
+
+import (
+	"strconv"
+
+	"cuelang.org/go/cue"
+	"cuelang.org/go/cue/ast"
+	"cuelang.org/go/cue/errors"
+)
+
+// Diff returns an edit script representing the difference between x and y.
+func Diff(x, y cue.Value) (Kind, *EditScript) {
+	d := differ{}
+	return d.diffValue(x, y)
+}
+
+// Kind identifies the kind of operation of an edit script.
+type Kind uint8
+
+const (
+	// Identity indicates that a value pair is identical in both list X and Y.
+	Identity Kind = iota
+	// UniqueX indicates that a value only exists in X and not Y.
+	UniqueX
+	// UniqueY indicates that a value only exists in Y and not X.
+	UniqueY
+	// Modified indicates that a value pair is a modification of each other.
+	Modified
+)
+
+// EditScript represents the series of differences between two CUE values.
+// x and y must be either both list or struct.
+type EditScript struct {
+	x, y  cue.Value
+	edits []Edit
+}
+
+// Len returns the number of edits.
+func (es *EditScript) Len() int {
+	return len(es.edits)
+}
+
+// Label returns a string representation of the label.
+//
+func (es *EditScript) LabelX(i int) string {
+	e := es.edits[i]
+	p := e.XPos()
+	if p < 0 {
+		return ""
+	}
+	return label(es.x, p)
+}
+
+func (es *EditScript) LabelY(i int) string {
+	e := es.edits[i]
+	p := e.YPos()
+	if p < 0 {
+		return ""
+	}
+	return label(es.y, p)
+}
+
+// TODO: support label expressions.
+func label(v cue.Value, i int) string {
+	st, err := v.Struct()
+	if err != nil {
+		return ""
+	}
+
+	// TODO: return formatted expression for optionals.
+	f := st.Field(i)
+	str := f.Name
+	if !ast.IsValidIdent(str) {
+		str = strconv.Quote(str)
+	}
+	if f.IsOptional {
+		str += "?"
+	}
+	if f.IsDefinition {
+		str += " ::"
+	} else {
+		str += ":"
+	}
+	return str
+}
+
+// ValueX returns the value of X involved at step i.
+func (es *EditScript) ValueX(i int) (v cue.Value) {
+	p := es.edits[i].XPos()
+	if p < 0 {
+		return v
+	}
+	st, err := es.x.Struct()
+	if err != nil {
+		return v
+	}
+	return st.Field(p).Value
+}
+
+// ValueY returns the value of Y involved at step i.
+func (es *EditScript) ValueY(i int) (v cue.Value) {
+	p := es.edits[i].YPos()
+	if p < 0 {
+		return v
+	}
+	st, err := es.y.Struct()
+	if err != nil {
+		return v
+	}
+	return st.Field(p).Value
+}
+
+// Edit represents a single operation within an edit-script.
+type Edit struct {
+	kind Kind
+	xPos int32       // 0 if UniqueY
+	yPos int32       // 0 if UniqueX
+	sub  *EditScript // non-nil if Modified
+}
+
+func (e Edit) Kind() Kind { return e.kind }
+func (e Edit) XPos() int  { return int(e.xPos - 1) }
+func (e Edit) YPos() int  { return int(e.yPos - 1) }
+
+type differ struct {
+	options []cue.Option
+	errs    errors.Error
+}
+
+func (d *differ) diffValue(x, y cue.Value) (Kind, *EditScript) {
+	if x.IncompleteKind() != y.IncompleteKind() {
+		return Modified, nil
+	}
+
+	switch xc, yc := x.IsConcrete(), y.IsConcrete(); {
+	case xc != yc:
+		return Modified, nil
+
+	case xc && yc:
+		switch k := x.Kind(); k {
+		case cue.StructKind:
+			return d.diffStruct(x, y)
+
+		case cue.ListKind:
+			return d.diffList(x, y)
+		}
+		fallthrough
+
+	default:
+		if !x.Equals(y) {
+			return Modified, nil
+		}
+	}
+
+	return Identity, nil
+}
+
+func (d *differ) diffStruct(x, y cue.Value) (Kind, *EditScript) {
+	sx, _ := x.Struct()
+	sy, _ := y.Struct()
+
+	// Best-effort topological sort, prioritizing x over y, using a variant of
+	// Kahn's algorithm (see, for instance
+	// https://www.geeksforgeeks.org/topological-sorting-indegree-based-solution/).
+	// We assume that the order of the elements of each value indicate an edge
+	// in the graph. This means that only the next unprocessed nodes can be
+	// those with no incoming edges.
+	xMap := make(map[string]int32, sx.Len())
+	yMap := make(map[string]int32, sy.Len())
+	for i := 0; i < sx.Len(); i++ {
+		xMap[sx.Field(i).Name] = int32(i + 1)
+	}
+	for i := 0; i < sy.Len(); i++ {
+		yMap[sy.Field(i).Name] = int32(i + 1)
+	}
+
+	edits := []Edit{}
+	differs := false
+
+	var xi, yi int
+	var xf, yf cue.FieldInfo
+	for xi < sx.Len() || yi < sx.Len() {
+		// Process zero nodes
+		for ; xi < sx.Len(); xi++ {
+			xf = sx.Field(xi)
+			yp := yMap[xf.Name]
+			if yp > 0 {
+				break
+			}
+			edits = append(edits, Edit{UniqueX, int32(xi + 1), 0, nil})
+			differs = true
+		}
+		for ; yi < sy.Len(); yi++ {
+			yf = sy.Field(yi)
+			if yMap[yf.Name] == 0 {
+				// already done
+				continue
+			}
+			xp := xMap[yf.Name]
+			if xp > 0 {
+				break
+			}
+			yMap[yf.Name] = 0
+			edits = append(edits, Edit{UniqueY, 0, int32(yi + 1), nil})
+			differs = true
+		}
+
+		// Compare nodes
+		for ; xi < sx.Len(); xi++ {
+			xf = sx.Field(xi)
+
+			yp := yMap[xf.Name]
+			if yp == 0 {
+				break
+			}
+			// If yp != xi+1, the topological sort was not possible.
+			yMap[xf.Name] = 0
+
+			yf := sy.Field(int(yp - 1))
+
+			kind := Identity
+			var script *EditScript
+			switch {
+			case xf.IsDefinition != yf.IsDefinition,
+				xf.IsOptional != yf.IsOptional:
+				kind = Modified
+
+			default:
+				xv := xf.Value
+				yv := yf.Value
+				// TODO(perf): consider evaluating lazily.
+				kind, script = d.diffValue(xv, yv)
+			}
+
+			edits = append(edits, Edit{kind, int32(xi + 1), yp, script})
+			if kind != Identity {
+				differs = true
+			}
+		}
+	}
+	if !differs {
+		return Identity, nil
+	}
+	return Modified, &EditScript{x: x, y: y, edits: edits}
+}
+
+// TODO: right now we do a simple element-by-element comparison. Instead,
+// use an algorithm that approximates a minimal Levenshtein distance, like the
+// one in github.com/google/go-cmp/internal/diff.
+func (d *differ) diffList(x, y cue.Value) (Kind, *EditScript) {
+	ix, _ := x.List()
+	iy, _ := y.List()
+
+	edits := []Edit{}
+	differs := false
+	i := int32(1)
+
+	for {
+		// TODO: This would be much easier with a Next/Done API.
+		hasX := ix.Next()
+		hasY := iy.Next()
+		if !hasX {
+			for hasY {
+				differs = true
+				edits = append(edits, Edit{UniqueY, 0, i, nil})
+				hasY = iy.Next()
+				i++
+			}
+			break
+		}
+		if !hasY {
+			for hasX {
+				differs = true
+				edits = append(edits, Edit{UniqueX, i, 0, nil})
+				hasX = ix.Next()
+				i++
+			}
+			break
+		}
+
+		// Both x and y have a value.
+		kind, script := d.diffValue(ix.Value(), iy.Value())
+		if kind != Identity {
+			differs = true
+		}
+		edits = append(edits, Edit{kind, i, i, script})
+		i++
+	}
+	if !differs {
+		return Identity, nil
+	}
+	return Modified, &EditScript{x: x, y: y, edits: edits}
+}
diff --git a/internal/diff/diff_test.go b/internal/diff/diff_test.go
new file mode 100644
index 0000000..e33093a
--- /dev/null
+++ b/internal/diff/diff_test.go
@@ -0,0 +1,359 @@
+// Copyright 2019 CUE Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package diff
+
+import (
+	"bytes"
+	"testing"
+
+	"cuelang.org/go/cue"
+)
+
+func TestDiff(t *testing.T) {
+	testCases := []struct {
+		name string
+		x, y string
+		kind Kind
+		diff string
+	}{{
+		name: "identity struct",
+		x: `{
+			a: {
+				b: 1
+				c: 2
+			}
+			l: {
+				d: 1
+			}
+		}`,
+		y: `{
+			a: {
+				c: 2
+				b: 1
+			}
+			l: {
+				d: 1
+			}
+		}`,
+	}, {
+		name: "identity list",
+		x:    `[1, 2, 3]`,
+		y:    `[1, 2, 3]`,
+	}, {
+		name: "identity value",
+		x:    `"foo"`,
+		y:    `"foo"`,
+	}, {
+		name: "modified value",
+		x:    `"foo"`,
+		y:    `"bar"`,
+		kind: Modified,
+	}, {
+		name: "basics",
+		x: `{
+			a: int
+			b: 2
+			s: 4
+			d: 1
+		}
+		`,
+		y: `
+		{
+			a: string
+			c: 3
+			s: 4
+			d: int
+		}
+		`,
+		kind: Modified,
+		diff: `  {
+-     a: int
++     a: string
+-     b: 2
+      s: 4
+-     d: 1
++     d: int
++     c: 3
+  }
+`,
+	}, {
+		name: "basics 2",
+		x: `{
+			ls: [2, 3, 4]
+			"foo-bar": 2
+			s: 4
+			lm1: [2, 3, 5]
+			lm2: [6]
+		}
+		`,
+		y: `
+		{
+			ls: [2, 3, 4]
+			"foo-bar": 3
+			s: 4
+			lm1: [2, 3, 4, 6]
+			lm2: []
+			la: [2, 3, 4]
+		}
+		`,
+		kind: Modified,
+		diff: `  {
+      ls: [2, 3, 4]
+-     "foo-bar": 2
++     "foo-bar": 3
+      s: 4
+      lm1: [
+          2,
+          3,
+-         5,
++         4,
++         6,
+      ]
+      lm2: [
+-         6,
+      ]
++     la: [2, 3, 4]
+  }
+`,
+	}, {
+		name: "interupted run 1",
+		x: `{
+	a: 1
+	b: 2
+	c: 3
+	d: 4
+	e: 10
+	f: 6
+	g: 7
+	h: 8
+	i: 9
+	j: 10
+}
+`,
+		y: `
+{
+	a: 1
+	b: 2
+	c: 3
+	d: 4
+	e: 5
+	f: 6
+	g: 7
+	h: 8
+	i: 9
+	j: 10
+}
+`,
+		kind: Modified,
+		diff: `  {
+      ... // 2 identical elements
+      c: 3
+      d: 4
+-     e: 10
++     e: 5
+      f: 6
+      g: 7
+      ... // 3 identical elements
+  }
+`,
+	}, {
+		name: "interupted run 2",
+		x: `{
+			a: -1
+			b: 2
+			c: 3
+			d: 4
+			e: 5
+			f: 6
+			g: 7
+			h: 8
+			i: 9
+			j: -10
+		}`,
+		y: `{
+			a: 1
+			b: 2
+			c: 3
+			d: 4
+			e: 5
+			f: 6
+			g: 7
+			h: 8
+			i: 9
+			j: 10
+		}
+		`,
+		kind: Modified,
+		diff: `  {
+-     a: -1
++     a: 1
+      b: 2
+      c: 3
+      ... // 4 identical elements
+      h: 8
+      i: 9
+-     j: -10
++     j: 10
+  }
+`,
+	}, {
+		name: "recursion",
+		x: `{
+		s: {
+			a: 1
+			b: 3
+			d: 4
+		}
+		l: [
+			[3, 4]
+		]
+	}`,
+		y: `{
+		s: {
+			a: 2
+			b: 3
+			c: 4
+		}
+		l: [
+			[3, 5, 6]
+		]
+	}
+	`,
+		kind: Modified,
+		diff: `  {
+      s: {
+-         a: 1
++         a: 2
+          b: 3
+-         d: 4
++         c: 4
+      }
+      l: [
+          [
+              3,
+-             4,
++             5,
++             6,
+          ]
+      ]
+  }
+`,
+	}, {
+		name: "optional and definitions",
+		x: `{
+	s :: {
+		a :: 1
+		b: 2
+	}
+	o?: 3
+	od? :: 1
+	oc?: 5
+}`,
+		y: `{
+	s :: {
+		a: 2
+		b :: 2
+	}
+	o?: 4
+	od :: 1
+	oc? :: 5
+}
+`,
+		kind: Modified,
+		diff: `  {
+      s :: {
+-         a :: 1
++         a: 2
+-         b: 2
++         b :: 2
+      }
+-     o?: 3
++     o?: 4
+-     od? :: 1
++     od :: 1
+-     oc?: 5
++     oc? :: 5
+  }
+`,
+	}}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			var r cue.Runtime
+			x, err := r.Compile("x", tc.x)
+			if err != nil {
+				t.Fatal(err)
+			}
+			y, err := r.Compile("y", tc.y)
+			if err != nil {
+				t.Fatal(err)
+			}
+			kind, script := Diff(x.Value(), y.Value())
+			if kind != tc.kind {
+				t.Fatalf("got %d; want %d", kind, tc.kind)
+			}
+			if script != nil {
+				w := &bytes.Buffer{}
+				err = Print(w, script)
+				if err != nil {
+					t.Fatal(err)
+				}
+				if got := w.String(); got != tc.diff {
+					t.Errorf("got\n%s;\nwant\n%s", got, tc.diff)
+				}
+			}
+		})
+	}
+}
+
+func TestX(t *testing.T) {
+	t.Skip()
+
+	tc := struct {
+		x, y string
+		kind Kind
+		diff string
+	}{
+		x: `{
+		}
+		`,
+		y: `
+		{
+		}
+		`,
+		kind: Modified,
+		diff: ``,
+	}
+	var r cue.Runtime
+	x, err := r.Compile("x", tc.x)
+	if err != nil {
+		t.Fatal(err)
+	}
+	y, err := r.Compile("y", tc.y)
+	if err != nil {
+		t.Fatal(err)
+	}
+	kind, script := Diff(x.Value(), y.Value())
+	if kind != tc.kind {
+		t.Fatalf("got %d; want %d", kind, tc.kind)
+	}
+	w := &bytes.Buffer{}
+	err = Print(w, script)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := w.String(); got != tc.diff {
+		t.Errorf("got\n%s;\nwant\n%s", got, tc.diff)
+	}
+}
diff --git a/internal/diff/print.go b/internal/diff/print.go
new file mode 100644
index 0000000..ebe2d10
--- /dev/null
+++ b/internal/diff/print.go
@@ -0,0 +1,271 @@
+// Copyright 2019 CUE Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package diff
+
+import (
+	"fmt"
+	"io"
+
+	"cuelang.org/go/cue"
+	"cuelang.org/go/cue/errors"
+	"cuelang.org/go/cue/format"
+)
+
+// Print the differences between two structs represented by an edit script.
+func Print(w io.Writer, es *EditScript) error {
+	p := printer{
+		w:       w,
+		margin:  2,
+		context: 2,
+	}
+	p.script(es)
+	return p.errs
+}
+
+type printer struct {
+	w         io.Writer
+	context   int
+	margin    int
+	indent    int
+	prefix    string
+	hasPrefix bool
+	hasPrint  bool
+	errs      errors.Error
+}
+
+func (p *printer) writeRaw(b []byte) {
+	if len(b) == 0 {
+		return
+	}
+	if !p.hasPrefix {
+		io.WriteString(p.w, p.prefix)
+		p.hasPrefix = true
+	}
+	if !p.hasPrint {
+		fmt.Fprintf(p.w, "% [1]*s", p.indent+p.margin-len(p.prefix), "")
+		p.hasPrint = true
+	}
+	p.w.Write(b)
+}
+
+func (p *printer) Write(b []byte) (n int, err error) {
+	i, last := 0, 0
+	for ; i < len(b); i++ {
+		if b[i] != '\n' {
+			continue
+		}
+		p.writeRaw(b[last:i])
+		last = i + 1
+		io.WriteString(p.w, "\n")
+		p.hasPrefix = false
+		p.hasPrint = false
+	}
+	p.writeRaw(b[last:])
+	return len(b), nil
+}
+
+func (p *printer) write(b []byte) {
+	_, _ = p.Write(b)
+}
+
+func (p *printer) printLen(align int, str string) {
+	fmt.Fprintf(p, "% -[1]*s", align, str)
+}
+
+func (p *printer) println(s string) {
+	fmt.Fprintln(p, s)
+}
+
+func (p *printer) printf(format string, args ...interface{}) {
+	fmt.Fprintf(p, format, args...)
+}
+
+func (p *printer) script(e *EditScript) {
+	switch e.x.Kind() {
+	case cue.StructKind:
+		p.printStruct(e)
+	case cue.ListKind:
+		p.printList(e)
+	default:
+		p.printf("BadExpr")
+	}
+}
+
+func (p *printer) findRun(es *EditScript, i int) (start, end int) {
+	lastEnd := i
+
+	for ; i < es.Len() && es.edits[i].kind == Identity; i++ {
+	}
+	start = i
+
+	// Find end of run
+	include := p.context
+	for ; i < es.Len(); i++ {
+		e := es.edits[i]
+		if e.kind != Identity {
+			include = p.context + 1
+			continue
+		}
+		if include--; include == 0 {
+			break
+		}
+	}
+
+	if i-start > 0 {
+		// Adjust start of run
+		if s := start - p.context; s > lastEnd {
+			start = s
+		} else {
+			start = lastEnd
+		}
+	}
+	return start, i
+}
+
+func (p *printer) printStruct(es *EditScript) {
+	// TODO: consider not printing outer curlies, or make it an option.
+	// if p.indent > 0 {
+	p.println("{")
+	defer p.println("}")
+	// }
+	p.indent += 4
+	defer func() {
+		p.indent -= 4
+	}()
+
+	var start, i int
+	for i < es.Len() {
+		lastEnd := i
+		// Find provisional start of run.
+		start, i = p.findRun(es, i)
+
+		p.printSkipped(start - lastEnd)
+		p.printFieldRun(es, start, i)
+	}
+	p.printSkipped(es.Len() - i)
+}
+
+func (p *printer) printList(es *EditScript) {
+	p.println("[")
+	p.indent += 4
+	defer func() {
+		p.indent -= 4
+		p.println("]")
+	}()
+
+	x := getElems(es.x)
+	y := getElems(es.y)
+
+	var start, i int
+	for i < es.Len() {
+		lastEnd := i
+		// Find provisional start of run.
+		start, i = p.findRun(es, i)
+
+		p.printSkipped(start - lastEnd)
+		p.printElemRun(es, x, y, start, i)
+	}
+	p.printSkipped(es.Len() - i)
+}
+
+func getElems(x cue.Value) (a []cue.Value) {
+	for i, _ := x.List(); i.Next(); {
+		a = append(a, i.Value())
+	}
+	return a
+}
+
+func (p *printer) printSkipped(n int) {
+	if n > 0 {
+		p.printf("... // %d identical elements\n", n)
+	}
+}
+
+func (p *printer) printValue(v cue.Value) {
+	// TODO: have indent option.
+	b, _ := format.Node(v.Syntax())
+	p.write(b)
+}
+
+func (p *printer) printFieldRun(es *EditScript, start, end int) {
+	// Determine max field len.
+	for i := start; i < end; i++ {
+		e := es.edits[i]
+
+		switch e.kind {
+		case UniqueX:
+			p.printField("-", es, es.LabelX(i), es.ValueX(i))
+
+		case UniqueY:
+			p.printField("+", es, es.LabelY(i), es.ValueY(i))
+
+		case Modified:
+			if e.sub != nil {
+				io.WriteString(p, es.LabelX(i))
+				io.WriteString(p, " ")
+				p.script(e.sub)
+				break
+			}
+			// TODO: show per-line differences for multiline strings.
+			p.printField("-", es, es.LabelX(i), es.ValueX(i))
+			p.printField("+", es, es.LabelY(i), es.ValueY(i))
+
+		case Identity:
+			// TODO: write on one line
+			p.printField("", es, es.LabelX(i), es.ValueX(i))
+		}
+	}
+}
+
+func (p *printer) printField(prefix string, es *EditScript, label string, v cue.Value) {
+	p.prefix = prefix
+	io.WriteString(p, label)
+	io.WriteString(p, " ")
+	p.printValue(v)
+	io.WriteString(p, "\n")
+	p.prefix = ""
+}
+
+func (p *printer) printElemRun(es *EditScript, x, y []cue.Value, start, end int) {
+	for _, e := range es.edits[start:end] {
+		switch e.kind {
+		case UniqueX:
+			p.printElem("-", x[e.XPos()])
+
+		case UniqueY:
+			p.printElem("+", y[e.YPos()])
+
+		case Modified:
+			if e.sub != nil {
+				p.script(e.sub)
+				break
+			}
+			// TODO: show per-line differences for multiline strings.
+			p.printElem("-", x[e.XPos()])
+			p.printElem("+", y[e.YPos()])
+
+		case Identity:
+			// TODO: write on one line
+			p.printElem("", x[e.XPos()])
+		}
+	}
+}
+
+func (p *printer) printElem(prefix string, v cue.Value) {
+	p.prefix = prefix
+	p.printValue(v)
+	io.WriteString(p, ",\n")
+	p.prefix = ""
+}