diff --git a/README.md b/README.md
index e29a585..3afea73 100644
--- a/README.md
+++ b/README.md
@@ -102,7 +102,7 @@ A validation library for TypeScript needs to be:
   - Common validators are available
 - Lightweight
   - Dependency free
-  - Tiny bundle size (~ 10Kb)
+  - Tiny bundle size
   - Fully tree shakeable
 
 `reviewed` exposes a flexible interface that achieves these goals.
@@ -245,6 +245,30 @@ if (guard(isNumber)(input)) {
 }
 ```
 
+### Optional fields
+
+Validators can be made optional:
+
+```ts
+interface Person {
+  name?: string;
+}
+
+const isPerson = isRecordOf<Person>({ name: optional(isString) });
+
+isPerson({}) >>
+  {
+    valid: true,
+    parsed: { name: "Joel" },
+  };
+
+isPerson({ name: "Joel" }) >>
+  {
+    valid: true,
+    parsed: { name: "Joel" },
+  };
+```
+
 ### Using literals
 
 Array literals can be converted directly to validators:
@@ -327,6 +351,8 @@ const not: <T>(validator: Validator<T>, reason?: string) => Validator<T>;
 const both: <T, U>(first: Validator<T>, second: Validator<U>) => Validator<T & U>;
 const either: <T, U>(first: Validator<T>, second: Validator<U>) => Validator<T | U>;
 
+const optional = <T>(validator: Validator<T>): Validator<T | undefined>;
+
 const isArrayOf: <T>(validator: Validator<T>) => Validator<T[]>;
 const isRecordOf: <T>(validators: ValidatorFields<T>) => Validator<T>;
 ```
diff --git a/src/factories/transform.spec.ts b/src/factories/transform.spec.ts
index 5bd4793..161a5a7 100644
--- a/src/factories/transform.spec.ts
+++ b/src/factories/transform.spec.ts
@@ -1,4 +1,5 @@
-import { both, either, not } from "./transform";
+import { both, either, not, optional } from "./transform";
+import { isNaturalNumberString } from "../validators/strings";
 import { isNonEmptyArray, isStringArray } from "../validators/arrays";
 import { isNull, isString } from "../validators/primitives";
 
@@ -52,3 +53,15 @@ describe("either", () => {
     expect(isStringOrNull(1).error).toBe("Not a string: 1");
   });
 });
+
+describe("optional", () => {
+  it("allows a validator to accept undefined inputs", () => {
+    expect(optional(isNaturalNumberString)(1).valid).toBe(false);
+
+    expect(optional(isNaturalNumberString)("1").valid).toBe(true);
+    expect(optional(isNaturalNumberString)("1").parsed).toBe(1);
+
+    expect(optional(isNaturalNumberString)(undefined).valid).toBe(true);
+    expect(optional(isNaturalNumberString)(undefined).parsed).toBe(undefined);
+  });
+});
diff --git a/src/factories/transform.ts b/src/factories/transform.ts
index 06674c4..5096f30 100644
--- a/src/factories/transform.ts
+++ b/src/factories/transform.ts
@@ -1,5 +1,6 @@
 import { Validated, Validator } from "../models/validators";
 import { invalidateWith } from "./invalidate";
+import { isUndefined } from "../validators/primitives";
 import { validate } from "./validate";
 
 /**
@@ -114,3 +115,32 @@ export const either =
 
     return right.valid ? right : left;
   };
+
+/**
+ * Allow a validator to accept undefined inputs
+ *
+ * @category Factories
+ * @example
+ *   interface Person {
+ *     name?: string;
+ *   }
+ *
+ *   const isPerson = isRecordOf<Person>({ name: optional(isString) });
+ *
+ *   isPerson({}) >>
+ *     {
+ *       valid: true,
+ *       parsed: { name: undefined },
+ *     };
+ *
+ *   isPerson({ name: "Joel" }) >>
+ *     {
+ *       valid: true,
+ *       parsed: { name: "Joel" },
+ *     };
+ *
+ * @typeParam T - The validated type
+ * @param first  - The validator
+ */
+export const optional = <T>(validator: Validator<T>): Validator<T | undefined> =>
+  either(validator, isUndefined);
diff --git a/src/factories/validate.spec.ts b/src/factories/validate.spec.ts
index 2ed95cd..c66591c 100644
--- a/src/factories/validate.spec.ts
+++ b/src/factories/validate.spec.ts
@@ -1,4 +1,6 @@
+import { isNaturalNumberString } from "../validators/strings";
 import { isNumber, isString } from "../validators/primitives";
+import { optional } from "./transform";
 import { validate, validateWith, validateWithAtLeast } from "./validate";
 
 describe("validate", () => {
@@ -62,6 +64,22 @@ describe("validateWith", () => {
       error: "Unexpected extra fields: c",
     });
   });
+
+  it("parses optional fields", () => {
+    const validator = validateWith<{ a: string; b?: number }>({
+      a: isString,
+      b: optional(isNaturalNumberString),
+    });
+
+    expect(validator({ a: "1" }).valid).toBe(true);
+    expect(validator({ a: "1" }).parsed).toEqual({ a: "1", b: undefined });
+
+    expect(validator({ a: "1", b: undefined }).valid).toBe(true);
+    expect(validator({ a: "1", b: undefined }).parsed).toEqual({ a: "1", b: undefined });
+
+    expect(validator({ a: "1", b: 1 }).error).toEqual({ b: "Not a string: 1" });
+    expect(validator({ a: "1", b: "1" }).parsed).toEqual({ a: "1", b: 1 });
+  });
 });
 
 describe("validateWithAtLeast", () => {
@@ -115,4 +133,20 @@ describe("validateWithAtLeast", () => {
       error: null,
     });
   });
+
+  it("parses optional fields", () => {
+    const validator = validateWithAtLeast<{ a: string; b?: number }>({
+      a: isString,
+      b: optional(isNaturalNumberString),
+    });
+
+    expect(validator({ a: "1" }).valid).toBe(true);
+    expect(validator({ a: "1" }).parsed).toEqual({ a: "1", b: undefined });
+
+    expect(validator({ a: "1", b: undefined }).valid).toBe(true);
+    expect(validator({ a: "1", b: undefined }).parsed).toEqual({ a: "1", b: undefined });
+
+    expect(validator({ a: "1", b: 1 }).error).toEqual({ b: "Not a string: 1" });
+    expect(validator({ a: "1", b: "1" }).parsed).toEqual({ a: "1", b: 1 });
+  });
 });
diff --git a/src/factories/validate.ts b/src/factories/validate.ts
index 2a99939..6ce369c 100644
--- a/src/factories/validate.ts
+++ b/src/factories/validate.ts
@@ -63,7 +63,7 @@ export const validateWith =
     }
 
     const missing = Object.keys(validators).filter(
-      (i) => !(i in record.parsed),
+      (i) => !(i in record.parsed || validators[i as keyof T](undefined).valid),
     );
 
     if (missing.length > 0) {
@@ -111,7 +111,7 @@ export const validateWithAtLeast =
     }
 
     const missing = Object.keys(validators).filter(
-      (i) => !(i in record.parsed),
+      (i) => !(i in record.parsed || validators[i as keyof T](undefined).valid),
     );
 
     if (missing.length > 0) {