diff --git a/README.md b/README.md
index a77eaf8..233f7d4 100644
--- a/README.md
+++ b/README.md
@@ -240,23 +240,6 @@ Custom:  foo.bar%2Fqux=XYZ
 
 (`%5C` and `%2F` represent `\` and `/`, respectively.)
 
-Limitations
------------
-
- - Circular (self-referential) values are untested.
-
-Future Work
------------
-
-The following items would be nice to have in the future—though they are not being worked on yet:
-
- - An option to automatically treat all field names in `camelCase` or `underscore_case`.
- - Built-in support for the types in [`math/big`](http://golang.org/pkg/math/big/).
- - Built-in support for the types in [`image/color`](http://golang.org/pkg/image/color/).
- - Improve encoding/decoding by reading/writing directly from/to the `io.Reader`/`io.Writer` when possible, rather than going through an intermediate representation (i.e. `node`) which requires more memory.
-
-(Feel free to implement any of these and then send a pull request.)
-
 Related Work
 ------------
 
diff --git a/TODO.md b/TODO.md
index d344727..76c7107 100644
--- a/TODO.md
+++ b/TODO.md
@@ -2,3 +2,7 @@ TODO
 ====
 
   - Document IgnoreCase and IgnoreUnknownKeys in README.
+  - An option to automatically treat all field names in `camelCase` or `underscore_case`.
+  - Built-in support for the types in [`math/big`](http://golang.org/pkg/math/big/).
+  - Built-in support for the types in [`image/color`](http://golang.org/pkg/image/color/).
+  - Improve encoding/decoding by reading/writing directly from/to the `io.Reader`/`io.Writer` when possible, rather than going through an intermediate representation (i.e. `node`) which requires more memory.
diff --git a/encode.go b/encode.go
index de362a5..4c93f19 100644
--- a/encode.go
+++ b/encode.go
@@ -118,10 +118,11 @@ func encodeToNode(v reflect.Value, z bool, o bool) (n node, err error) {
 			err = fmt.Errorf("%v", e)
 		}
 	}()
-	return getNode(encodeValue(v, z, o)), nil
+	seen := make(map[uintptr]bool)
+	return getNode(encodeValue(v, z, o, seen)), nil
 }
 
-func encodeValue(v reflect.Value, z bool, o bool) interface{} {
+func encodeValue(v reflect.Value, z bool, o bool, seen map[uintptr]bool) interface{} {
 	t := v.Type()
 	k := v.Kind()
 
@@ -132,21 +133,35 @@ func encodeValue(v reflect.Value, z bool, o bool) interface{} {
 	}
 
 	switch k {
-	case reflect.Ptr, reflect.Interface:
-		return encodeValue(v.Elem(), z, o)
+	case reflect.Ptr:
+		ptr := v.Pointer()
+		if seen[ptr] {
+			panic("form: encoding a cycle via " + t.String())
+		}
+		seen[ptr] = true
+		defer delete(seen, ptr)
+		return encodeValue(v.Elem(), z, o, seen)
+	case reflect.Interface:
+		return encodeValue(v.Elem(), z, o, seen)
 	case reflect.Struct:
 		if t.ConvertibleTo(timeType) {
 			return encodeTime(v)
 		} else if t.ConvertibleTo(urlType) {
 			return encodeURL(v)
 		}
-		return encodeStruct(v, z, o)
+		return encodeStruct(v, z, o, seen)
 	case reflect.Slice:
-		return encodeSlice(v, z, o)
+		return encodeSlice(v, z, o, seen)
 	case reflect.Array:
-		return encodeArray(v, z, o)
+		return encodeArray(v, z, o, seen)
 	case reflect.Map:
-		return encodeMap(v, z, o)
+		ptr := v.Pointer()
+		if seen[ptr] {
+			panic("form: encoding a cycle via " + t.String())
+		}
+		seen[ptr] = true
+		defer delete(seen, ptr)
+		return encodeMap(v, z, o, seen)
 	case reflect.Invalid, reflect.Uintptr, reflect.UnsafePointer, reflect.Chan, reflect.Func:
 		panic(t.String() + " has unsupported kind " + t.Kind().String())
 	default:
@@ -160,7 +175,7 @@ type encoderField struct {
 	omitempty bool
 }
 
-func encodeStruct(v reflect.Value, z bool, o bool) interface{} {
+func encodeStruct(v reflect.Value, z bool, o bool, seen map[uintptr]bool) interface{} {
 	fields := collectFields(v.Type())
 	n := node{}
 	for _, f := range fields {
@@ -171,7 +186,7 @@ func encodeStruct(v reflect.Value, z bool, o bool) interface{} {
 		if (o || f.omitempty) && isEmptyValue(fv) {
 			continue
 		}
-		n[f.name] = encodeValue(fv, z, o)
+		n[f.name] = encodeValue(fv, z, o, seen)
 	}
 	return n
 }
@@ -326,31 +341,31 @@ func isLeafStruct(ft reflect.Type) bool {
 	return ft.Implements(textMarshalerType) || reflect.PtrTo(ft).Implements(textMarshalerType)
 }
 
-func encodeMap(v reflect.Value, z bool, o bool) interface{} {
+func encodeMap(v reflect.Value, z bool, o bool, seen map[uintptr]bool) interface{} {
 	n := node{}
 	for _, i := range v.MapKeys() {
-		k := getString(encodeValue(i, z, o))
-		n[k] = encodeValue(v.MapIndex(i), z, o)
+		k := getString(encodeValue(i, z, o, seen))
+		n[k] = encodeValue(v.MapIndex(i), z, o, seen)
 	}
 	return n
 }
 
-func encodeArray(v reflect.Value, z bool, o bool) interface{} {
+func encodeArray(v reflect.Value, z bool, o bool, seen map[uintptr]bool) interface{} {
 	n := node{}
 	for i := 0; i < v.Len(); i++ {
-		n[strconv.Itoa(i)] = encodeValue(v.Index(i), z, o)
+		n[strconv.Itoa(i)] = encodeValue(v.Index(i), z, o, seen)
 	}
 	return n
 }
 
-func encodeSlice(v reflect.Value, z bool, o bool) interface{} {
+func encodeSlice(v reflect.Value, z bool, o bool, seen map[uintptr]bool) interface{} {
 	t := v.Type()
 	if t.Elem().Kind() == reflect.Uint8 {
 		return string(v.Bytes()) // Encode byte slices as a single string by default.
 	}
 	n := node{}
 	for i := 0; i < v.Len(); i++ {
-		n[strconv.Itoa(i)] = encodeValue(v.Index(i), z, o)
+		n[strconv.Itoa(i)] = encodeValue(v.Index(i), z, o, seen)
 	}
 	return n
 }
diff --git a/encode_test.go b/encode_test.go
index 3246f33..6b79fb5 100644
--- a/encode_test.go
+++ b/encode_test.go
@@ -137,6 +137,46 @@ func TestEncode_OmitEmpty(t *testing.T) {
 	}
 }
 
+func TestEncode_Cycle(t *testing.T) {
+	t.Run("self-referential struct pointer", func(t *testing.T) {
+		type Cyclic struct {
+			Name string
+			Next *Cyclic
+		}
+		a := &Cyclic{Name: "a"}
+		a.Next = a
+
+		if _, err := EncodeToString(a); err == nil {
+			t.Error("expected error for cyclic struct pointer, got nil")
+		}
+	})
+
+	t.Run("map containing itself", func(t *testing.T) {
+		m := map[string]interface{}{}
+		m["self"] = m
+
+		if _, err := EncodeToString(m); err == nil {
+			t.Error("expected error for cyclic map, got nil")
+		}
+	})
+
+	t.Run("non-cyclic pointer sharing (DAG)", func(t *testing.T) {
+		type Node struct {
+			Value string
+		}
+		type DAG struct {
+			A *Node
+			B *Node
+		}
+		shared := &Node{Value: "shared"}
+		dag := DAG{A: shared, B: shared}
+
+		if _, err := EncodeToString(dag); err != nil {
+			t.Errorf("unexpected error for DAG: %s", err)
+		}
+	})
+}
+
 func TestEncode_ConflictResolution(t *testing.T) {
 	for _, c := range []struct {
 		name string