diff --git a/src/Either.php b/src/Either.php
index ec394a5..30e0aca 100644
--- a/src/Either.php
+++ b/src/Either.php
@@ -15,9 +15,21 @@ public function __construct(
         public mixed $result,
         public ?Throwable $error = null
     ) {
+        if ($result instanceof Throwable) {
+            throw new InvalidArgumentException('Result cannot be a Throwable.');
+        }
+        if ($result === null && $error === null) {
+            throw new InvalidArgumentException('Either result or error must be set.');
+        }
+
         parent::__construct(new ArrayIterator([$error, $result]));
     }
 
+    public static function create(mixed $result, ?Throwable $error = null): self
+    {
+        return new self($result, $error);
+    }
+
     public function getIterator(): Traversable
     {
         return new ArrayIterator([$this->error, $this->result]);
diff --git a/tests/Unit/EitherTest.php b/tests/Unit/EitherTest.php
new file mode 100644
index 0000000..4ad73f4
--- /dev/null
+++ b/tests/Unit/EitherTest.php
@@ -0,0 +1,96 @@
+<?php
+
+use MightFail\Either;
+
+test('class must exist', function () {
+    expect(class_exists(Either::class))->toBeTrue();
+});
+
+test('should extend the IteratorIterator class', function () {
+    $either = new Either('foo');
+
+    expect($either)->toBeInstanceOf(IteratorIterator::class);
+});
+
+test('should implement the ArrayAccess interface', function () {
+    $either = new Either('foo');
+
+    expect($either)->toBeInstanceOf(ArrayAccess::class);
+});
+
+test('should have a result property', function () {
+    $either = new Either('foo');
+
+    expect($either->result)->toBe('foo');
+});
+
+test('should have an error property', function () {
+    $either = new Either(null, new Exception('foo'));
+
+    expect($either->error)->toBeInstanceOf(Exception::class);
+});
+
+test('should throw an error if result is an exception', function () {
+    expect(fn () => new Either(new Exception('foo')))->toThrow(InvalidArgumentException::class, 'Result cannot be a Throwable.');
+});
+
+test('should throw an error if both result and error are null', function () {
+    expect(fn () => new Either(null, null))->toThrow(InvalidArgumentException::class, 'Either result or error must be set.');
+});
+
+test('should have the correct methods', function () {
+    expect(method_exists(Either::class, 'getIterator'))->toBeTrue()
+        ->and(method_exists(Either::class, 'offsetSet'))->toBeTrue()
+        ->and(method_exists(Either::class, 'offsetExists'))->toBeTrue()
+        ->and(method_exists(Either::class, 'offsetUnset'))->toBeTrue()
+        ->and(method_exists(Either::class, 'offsetGet'))->toBeTrue()
+        ->and(method_exists(Either::class, 'create'))->toBeTrue();
+});
+
+test('can be instantiated with new', function () {
+    $either = new Either('foo');
+
+    expect($either)->toBeInstanceOf(Either::class)
+        ->and($either->result)->toBe('foo')
+        ->and($either->error)->toBeNull();
+
+    [$error, $result] = $either;
+
+    expect($error)->toBeNull()
+        ->and($result)->toBe('foo');
+
+    $either = new Either(null, new Exception('foo'));
+
+    expect($either)->toBeInstanceOf(Either::class)
+        ->and($either->result)->toBeNull()
+        ->and($either->error)->toBeInstanceOf(Exception::class);
+
+    [$error, $result] = $either;
+
+    expect($error)->toBeInstanceOf(Exception::class)
+        ->and($result)->toBeNull();
+});
+
+test('can be instantiated with from', function () {
+    $either = Either::create('foo');
+
+    expect($either)->toBeInstanceOf(Either::class)
+        ->and($either->result)->toBe('foo')
+        ->and($either->error)->toBeNull();
+
+    [$error, $result] = $either;
+
+    expect($error)->toBeNull()
+        ->and($result)->toBe('foo');
+
+    $either = Either::create(null, new Exception('foo'));
+
+    expect($either)->toBeInstanceOf(Either::class)
+        ->and($either->result)->toBeNull()
+        ->and($either->error)->toBeInstanceOf(Exception::class);
+
+    [$error, $result] = $either;
+
+    expect($error)->toBeInstanceOf(Exception::class)
+        ->and($result)->toBeNull();
+});
diff --git a/tests/Unit/FailTest.php b/tests/Unit/FailTest.php
new file mode 100644
index 0000000..a76486b
--- /dev/null
+++ b/tests/Unit/FailTest.php
@@ -0,0 +1,44 @@
+<?php
+
+use MightFail\Either;
+use MightFail\Fail;
+use MightFail\Might;
+
+test('class must exist and have a static from method', function () {
+    expect(class_exists(Fail::class))->toBeTrue()
+        ->and(method_exists(Fail::class, 'from'))->toBeTrue();
+});
+
+test('creates a Fail instance', function () {
+    $might = Fail::from(new Exception('foo'));
+
+    expect($might)->toBeInstanceOf(Fail::class);
+});
+
+test('extends from the Either class', function () {
+    $might = Fail::from(new Exception('foo'));
+
+    expect($might)->toBeInstanceOf(Either::class)
+        ->and($might)->toBeInstanceOf(Fail::class)
+        ->and($might)->not->toBeInstanceOf(Might::class);
+});
+
+test('has Either properties', function () {
+    $might = Fail::from(new Exception('foo'));
+
+    expect($might->result)->toBeNull()
+        ->and($might->error)->toBeInstanceOf(Exception::class);
+
+    [$error, $result] = $might;
+
+    expect($error)->toBeInstanceOf(Exception::class)
+        ->and($result)->toBeNull();
+});
+
+test('can instantiate with new', function () {
+    $might = new Fail(new Exception('foo'));
+
+    expect($might)->toBeInstanceOf(Fail::class)
+        ->and($might->result)->toBeNull()
+        ->and($might->error)->toBeInstanceOf(Exception::class);
+});
diff --git a/tests/Unit/MightTest.php b/tests/Unit/MightTest.php
new file mode 100644
index 0000000..1c5a43b
--- /dev/null
+++ b/tests/Unit/MightTest.php
@@ -0,0 +1,44 @@
+<?php
+
+use MightFail\Either;
+use MightFail\Fail;
+use MightFail\Might;
+
+test('class must exist and have a static from method', function () {
+    expect(class_exists(Might::class))->toBeTrue()
+        ->and(method_exists(Might::class, 'from'))->toBeTrue();
+});
+
+test('creates a Might instance', function () {
+    $might = Might::from('foo');
+
+    expect($might)->toBeInstanceOf(Might::class);
+});
+
+test('extends from the Either class', function () {
+    $might = Might::from('foo');
+
+    expect($might)->toBeInstanceOf(Either::class)
+        ->and($might)->toBeInstanceOf(Might::class)
+        ->and($might)->not->toBeInstanceOf(Fail::class);
+});
+
+test('has Either properties', function () {
+    $might = Might::from('foo');
+
+    expect($might->result)->toBe('foo')
+        ->and($might->error)->toBeNull();
+
+    [$error, $result] = $might;
+
+    expect($error)->toBeNull()
+        ->and($result)->toBe('foo');
+});
+
+test('can instantiate with new', function () {
+    $might = new Might('foo');
+
+    expect($might)->toBeInstanceOf(Might::class)
+        ->and($might->result)->toBe('foo')
+        ->and($might->error)->toBeNull();
+});