diff --git a/README.md b/README.md
index 123e9c7..94944e3 100644
--- a/README.md
+++ b/README.md
@@ -78,6 +78,49 @@ const json = `{
 const data = devalue.unflatten(JSON.parse(json).data);
 ```
 
+## Custom types
+
+You can serialize and serialize custom types by passing a second argument to `stringify` containing an object of types and their _reducers_, and a second argument to `parse` or `unflatten` containing an object of types and their _revivers_:
+
+```js
+class Vector {
+	constructor(x, y) {
+		this.x = x;
+		this.y = y;
+	}
+
+	magnitude() {
+		return Math.sqrt(this.x * this.x + this.y * this.y);
+	}
+}
+
+const stringified = devalue.stringify(new Vector(30, 40), {
+	Vector: (value) => value instanceof Vector && [value.x, value.y]
+});
+
+console.log(stringified); // [["Vector",1],[2,3],30,40]
+
+const vector = devalue.parse(stringified, {
+	Vector: ([x, y]) => new Vector(x, y)
+});
+
+console.log(vector.magnitude()); // 50
+```
+
+If a function passed to `stringify` returns a truthy value, it's treated as a match.
+
+You can also use custom types with `uneval` by specifying a custom replacer:
+
+```js
+devalue.uneval(vector, (value, uneval) => {
+	if (value instanceof Vector) {
+		return `new Vector(${value.x},${value.y})`;
+	}
+}); // `new Vector(30,40)`
+```
+
+Note that any variables referenced in the resulting JavaScript (like `Vector` in the example above) must be in scope when it runs.
+
 ## Error handling
 
 If `uneval` or `stringify` encounters a function or a non-POJO, it will throw an error. You can find where in the input data the offending value lives by inspecting `error.path`:
diff --git a/src/parse.js b/src/parse.js
index 890bec3..54cdeba 100644
--- a/src/parse.js
+++ b/src/parse.js
@@ -10,16 +10,18 @@ import {
 /**
  * Revive a value serialized with `devalue.stringify`
  * @param {string} serialized
+ * @param {Record<string, (value: any) => any>} [revivers]
  */
-export function parse(serialized) {
-	return unflatten(JSON.parse(serialized));
+export function parse(serialized, revivers) {
+	return unflatten(JSON.parse(serialized), revivers);
 }
 
 /**
- * Revive a value flattened with `devalue.flatten`
+ * Revive a value flattened with `devalue.stringify`
  * @param {number | any[]} parsed
+ * @param {Record<string, (value: any) => any>} [revivers]
  */
-export function unflatten(parsed) {
+export function unflatten(parsed, revivers) {
 	if (typeof parsed === 'number') return hydrate(parsed, true);
 
 	if (!Array.isArray(parsed) || parsed.length === 0) {
@@ -30,7 +32,10 @@ export function unflatten(parsed) {
 
 	const hydrated = Array(values.length);
 
-	/** @param {number} index */
+	/**
+	 * @param {number} index
+	 * @returns {any}
+	 */
 	function hydrate(index, standalone = false) {
 		if (index === UNDEFINED) return undefined;
 		if (index === NAN) return NaN;
@@ -50,6 +55,11 @@ export function unflatten(parsed) {
 			if (typeof value[0] === 'string') {
 				const type = value[0];
 
+				const reviver = revivers?.[type];
+				if (reviver) {
+					return (hydrated[index] = reviver(hydrate(value[1])));
+				}
+
 				switch (type) {
 					case 'Date':
 						hydrated[index] = new Date(value[1]);
@@ -90,6 +100,9 @@ export function unflatten(parsed) {
 							obj[value[i]] = hydrate(value[i + 1]);
 						}
 						break;
+
+					default:
+						throw new Error(`Unknown type ${type}`);
 				}
 			} else {
 				const array = new Array(value.length);
diff --git a/src/stringify.js b/src/stringify.js
index 13adb5b..8ce427e 100644
--- a/src/stringify.js
+++ b/src/stringify.js
@@ -17,14 +17,21 @@ import {
 /**
  * Turn a value into a JSON string that can be parsed with `devalue.parse`
  * @param {any} value
+ * @param {Record<string, (value: any) => any>} [reducers]
  */
-export function stringify(value) {
+export function stringify(value, reducers) {
 	/** @type {any[]} */
 	const stringified = [];
 
 	/** @type {Map<any, number>} */
 	const indexes = new Map();
 
+	/** @type {Array<{ key: string, fn: (value: any) => any }>} */
+	const custom = [];
+	for (const key in reducers) {
+		custom.push({ key, fn: reducers[key] });
+	}
+
 	/** @type {string[]} */
 	const keys = [];
 
@@ -47,6 +54,14 @@ export function stringify(value) {
 		const index = p++;
 		indexes.set(thing, index);
 
+		for (const { key, fn } of custom) {
+			const value = fn(thing);
+			if (value) {
+				stringified[index] = `["${key}",${flatten(value)}]`;
+				return index;
+			}
+		}
+
 		let str = '';
 
 		if (is_primitive(thing)) {
diff --git a/src/uneval.js b/src/uneval.js
index 305fb6a..79f35a3 100644
--- a/src/uneval.js
+++ b/src/uneval.js
@@ -15,13 +15,16 @@ const reserved =
 /**
  * Turn a value into the JavaScript that creates an equivalent value
  * @param {any} value
+ * @param {(value: any) => string | void} [replacer]
  */
-export function uneval(value) {
+export function uneval(value, replacer) {
 	const counts = new Map();
 
 	/** @type {string[]} */
 	const keys = [];
 
+	const custom = new Map();
+
 	/** @param {any} thing */
 	function walk(thing) {
 		if (typeof thing === 'function') {
@@ -36,6 +39,15 @@ export function uneval(value) {
 
 			counts.set(thing, 1);
 
+			if (replacer) {
+				const str = replacer(thing);
+
+				if (typeof str === 'string') {
+					custom.set(thing, str);
+					return;
+				}
+			}
+
 			const type = get_type(thing);
 
 			switch (type) {
@@ -117,6 +129,10 @@ export function uneval(value) {
 			return stringify_primitive(thing);
 		}
 
+		if (custom.has(thing)) {
+			return custom.get(thing);
+		}
+
 		const type = get_type(thing);
 
 		switch (type) {
@@ -174,6 +190,11 @@ export function uneval(value) {
 		names.forEach((name, thing) => {
 			params.push(name);
 
+			if (custom.has(thing)) {
+				values.push(/** @type {string} */ (custom.get(thing)));
+				return;
+			}
+
 			if (is_primitive(thing)) {
 				values.push(stringify_primitive(thing));
 				return;
diff --git a/test/test.js b/test/test.js
index d440b41..1857c6b 100644
--- a/test/test.js
+++ b/test/test.js
@@ -3,6 +3,12 @@ import * as assert from 'uvu/assert';
 import * as uvu from 'uvu';
 import { uneval, unflatten, parse, stringify } from '../index.js';
 
+class Custom {
+	constructor(value) {
+		this.value = value;
+	}
+}
+
 const fixtures = {
 	basics: [
 		{
@@ -374,14 +380,39 @@ const fixtures = {
 				assert.equal(Object.keys(value).length, 0);
 			}
 		}
-	]
+	],
+
+	custom: ((instance) => [
+		{
+			name: 'Custom type',
+			value: [instance, instance],
+			js: '(function(a){return [a,a]}(new Custom({answer:42})))',
+			json: '[[1,1],["Custom",2],{"answer":3},42]',
+			replacer: (value) => {
+				if (value instanceof Custom) {
+					return `new Custom(${uneval(value.value)})`;
+				}
+			},
+			reducers: {
+				Custom: (x) => x instanceof Custom && x.value
+			},
+			revivers: {
+				Custom: (x) => new Custom(x)
+			},
+			validate: ([obj1, obj2]) => {
+				assert.is(obj1, obj2);
+				assert.ok(obj1 instanceof Custom);
+				assert.equal(obj1.value.answer, 42);
+			}
+		}
+	])(new Custom({ answer: 42 }))
 };
 
 for (const [name, tests] of Object.entries(fixtures)) {
 	const test = uvu.suite(`uneval: ${name}`);
 	for (const t of tests) {
 		test(t.name, () => {
-			const actual = uneval(t.value);
+			const actual = uneval(t.value, t.replacer);
 			const expected = t.js;
 			assert.equal(actual, expected);
 		});
@@ -393,7 +424,7 @@ for (const [name, tests] of Object.entries(fixtures)) {
 	const test = uvu.suite(`stringify: ${name}`);
 	for (const t of tests) {
 		test(t.name, () => {
-			const actual = stringify(t.value);
+			const actual = stringify(t.value, t.reducers);
 			const expected = t.json;
 			assert.equal(actual, expected);
 		});
@@ -405,7 +436,7 @@ for (const [name, tests] of Object.entries(fixtures)) {
 	const test = uvu.suite(`parse: ${name}`);
 	for (const t of tests) {
 		test(t.name, () => {
-			const actual = parse(t.json);
+			const actual = parse(t.json, t.revivers);
 			const expected = t.value;
 
 			if (t.validate) {
@@ -422,7 +453,7 @@ for (const [name, tests] of Object.entries(fixtures)) {
 	const test = uvu.suite(`unflatten: ${name}`);
 	for (const t of tests) {
 		test(t.name, () => {
-			const actual = unflatten(JSON.parse(t.json));
+			const actual = unflatten(JSON.parse(t.json), t.revivers);
 			const expected = t.value;
 
 			if (t.validate) {