diff --git a/CHANGELOG.md b/CHANGELOG.md
index f93cb10..457c593 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased] :bomb:
 
+## :bulb: [0.5.1] - 2020-03-23
+
+### :hammer: Fixed
+- `Either.chain` was fixed. The behaviour will be correct.
+
 ## :bulb: [0.5.0] - 2020-03-20
 
 ### :gift: Added
@@ -65,7 +70,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Smoke test and really build to **Travis CI**.
 - Labels of `npm`, `coverage`, `travis`.
 
-[unreleased]: https://github.com/snatvb/monad-maniac/compare/v0.5.0...develop
+[unreleased]: https://github.com/snatvb/monad-maniac/compare/v0.5.1...develop
+[0.5.1]: https://github.com/snatvb/monad-maniac/compare/v0.5.0...v0.5.1
 [0.5.0]: https://github.com/snatvb/monad-maniac/compare/v0.4.0...v0.5.0
 [0.4.0]: https://github.com/snatvb/monad-maniac/compare/v0.3.0...v0.4.0
 [0.3.0]: https://github.com/snatvb/monad-maniac/compare/v0.2.0...v0.3.0
diff --git a/src/either.ts b/src/either.ts
index f7807b6..21b8640 100644
--- a/src/either.ts
+++ b/src/either.ts
@@ -1,7 +1,7 @@
 import * as helpers from './helpers'
+import { Functor } from './interfaces'
 import * as Maybe from './maybe'
 import { Nullable } from './types'
-import { Functor } from './interfaces'
 
 /** Mather type for caseOf */
 export type CaseOf<L, R, U> = {
@@ -83,7 +83,7 @@ export interface Either<L, R> extends Functor<R> {
    * ```
    * @param f Function to apply for Right value
    */
-  chain<U>(f: (value: R) => U): U | Either<L, U>
+  chain<U>(f: (value: R) => Either<L, U>): Either<L, U>
   /**
    * Apply predicate function to value in container.
    * If the function returns not `true` then value from
@@ -433,15 +433,17 @@ export function map<L, R, U>(f: (value: R) => U, either?: Either<L, R>): Either<
  * const result: Either.Shape<string, string> = typeof name === 'string'
  *  ? new Either.Right(name)
  *  : new Either.Left('Server error')
- * const greeting = Either.chain((name) => `Welcome, ${name}!`, result) // 'Welcome, Jake' or 'Server error'
+ * const greeting = Either.chain(
+ *    (name: string) => Either.right(`Welcome, ${name}!`)
+ * ), result).get() // 'Welcome, Jake' or 'Server error'
  * ```
  * */
-export function chain<L, R, U>(f: (value: R) => U, either: Either<L, R>): Either<L, U> | U
+export function chain<L, R, U>(f: (value: R) => Either<L, U>, either: Either<L, R>): Either<L, U>
 /**
  * Just curried `chain`.
  */
-export function chain<L, R, U>(f: (value: R) => U): (either: Either<L, R>) => Either<L, U> | U
-export function chain<L, R, U>(f: (value: R) => U, either?: Either<L, R>): Either<L, U> | U | ((either: Either<L, R>) => Either<L, U> | U) {
+export function chain<L, R, U>(f: (value: R) => Either<L, U>): (either: Either<L, R>) => Either<L, U>
+export function chain<L, R, U>(f: (value: R) => Either<L, U>, either?: Either<L, R>): Either<L, U> | ((either: Either<L, R>) => Either<L, U>) {
   const op = (either: Either<L, R>) => either.chain(f)
   return helpers.curry1(op, either)
 }
@@ -580,7 +582,7 @@ export class Right<L ,R> implements Either<L ,R> {
     return this.value
   }
 
-  chain<U>(f: (value: R) => U): U | Either<L, U> {
+  chain<U>(f: (value: R) => Either<L, U>): Either<L, U> {
     return f(this.value)
   }
 
@@ -628,7 +630,7 @@ export class Left<L, R> implements  Either<L, R> {
     return f(this.value)
   }
 
-  chain<U>(_f: (value: R) => U): U | Either<L, U> {
+  chain<U>(_f: (value: R) => Either<L, U>): Either<L, U> {
     return new Left<L, U>(this.value)
   }
 
diff --git a/tests/either.test.ts b/tests/either.test.ts
index 7ceaab4..72ee989 100644
--- a/tests/either.test.ts
+++ b/tests/either.test.ts
@@ -1,6 +1,8 @@
 import { Either } from '../src'
 
 const double = (x: number): number => x * 2
+const chainDouble = (x: number): Either.Shape<string, number> => Either.right(x * 2)
+const greetChain = (name: string): Either.Shape<string, string> => Either.right<string, string>(`Welcome, ${name}!`)
 
 describe('Either pure functions', () => {
   describe('of', () => {
@@ -84,19 +86,19 @@ describe('Either pure functions', () => {
     it('direct call', () => {
       const left: Either.Shape<string, number> = new Either.Left('Server error')
       const right: Either.Shape<string, number> = new Either.Right(150)
-      const leftResult = Either.chain(double, left)
+      const leftResult = Either.chain(chainDouble, left)
 
       expect(leftResult instanceof Either.Left).toBe(true)
-      expect(Either.chain(double, right)).toBe(300)
+      expect(Either.chain(chainDouble, right).get()).toBe(300)
     })
 
     it('carried', () => {
       const left: Either.Shape<string, number> = new Either.Left('Server error')
       const right: Either.Shape<string, number> = new Either.Right(150)
-      const leftResult = Either.chain(double)(left)
+      const leftResult = Either.chain(chainDouble)(left)
 
       expect(leftResult instanceof Either.Left).toBe(true)
-      expect(Either.chain(double)(right)).toBe(300)
+      expect(Either.chain(chainDouble)(right).get()).toBe(300)
     })
   })
 
@@ -280,8 +282,8 @@ describe('Either: Left & Right', () => {
     const left: Either.Shape<string, number> = new Either.Left('Server error')
     const right: Either.Shape<string, number> = new Either.Right(150)
 
-    expect(left.chain(double) instanceof Either.Left).toBe(true)
-    expect(right.chain(double)).toBe(300)
+    expect(left.chain(chainDouble).isLeft()).toBe(true)
+    expect(right.chain(chainDouble).isRight()).toBe(true)
   })
 
   it('getOrElse', () => {
@@ -344,10 +346,10 @@ describe('Either: Left & Right', () => {
 describe('Cases from docs', () => {
   it('map', () => {
     const divide = (dividend: number) => (divider: number): Either.Shape<string, number> => {
-     if (divider === 0) {
-       return Either.left('Divider is zero!')
-     }
-     return Either.right(dividend / divider)
+      if (divider === 0) {
+        return Either.left('Divider is zero!')
+      }
+      return Either.right(dividend / divider)
     }
 
     const resultNormal = divide(10)(5).map(double).get() // 4
@@ -358,10 +360,10 @@ describe('Cases from docs', () => {
 
   it('chain', () => {
     const divide = (dividend: number) => (divider: number): Either.Shape<string, number> => {
-     if (divider === 0) {
-       return Either.left('Divider is zero!')
-     }
-     return Either.right(dividend / divider)
+      if (divider === 0) {
+        return Either.left('Divider is zero!')
+      }
+      return Either.right(dividend / divider)
     }
 
     const nonZeroMultiply = (multiplicand: number) => (factor: number): Either.Shape<string, number> => {
@@ -471,4 +473,16 @@ describe('Cases from docs', () => {
     expect(resultRight.toString()).toBe('Just(150)')
     expect(resultRightVoid.toString()).toBe('Nothing()')
   })
+
+  // ========= DOC TESTS =========
+  it('Chain from doc', () => {
+
+    const greeting = Either.chain(greetChain, Either.right('Jake')).get()
+    expect(greeting).toBe('Welcome, Jake!')
+
+    // Server error
+
+    const serverError = Either.chain(greetChain, Either.left('Server error')).get()
+    expect(serverError).toBe('Server error')
+  })
 })