diff --git a/README.md b/README.md
index 535fc44..7e0a922 100644
--- a/README.md
+++ b/README.md
@@ -204,6 +204,8 @@ All Result types (both Ok and Err) implement these methods:
 #### Value Extraction
 - `unwrap(): mixed` - Returns the success value or throws UnwrapException (extends LogicException)
 - `unwrapErr(): mixed` - Returns the error value or throws UnwrapException (extends LogicException)
+- `expect(string $message): mixed` - Returns the success value or throws UnwrapException with the given message and a summary of the error value
+- `expectErr(string $message): mixed` - Returns the error value or throws UnwrapException with the given message and a summary of the success value
 - `unwrapOr(mixed $default): mixed` - Returns the success value or a default
 - `unwrapOrElse(callable $fn): mixed` - Returns the success value or computes it from the error
 
diff --git a/src/Err.php b/src/Err.php
index ade25ab..476c171 100644
--- a/src/Err.php
+++ b/src/Err.php
@@ -62,6 +62,21 @@ public function unwrapErr(): mixed
         return $this->value;
     }
 
+    #[Override]
+    public function expect(string $message): never
+    {
+        throw UnwrapException::withMessage($message, $this->value);
+    }
+
+    /**
+     * @return E
+     */
+    #[Override]
+    public function expectErr(string $message): mixed
+    {
+        return $this->value;
+    }
+
     /**
      * @template U
      * @param U $default
diff --git a/src/Ok.php b/src/Ok.php
index d494672..865ee7c 100644
--- a/src/Ok.php
+++ b/src/Ok.php
@@ -62,6 +62,21 @@ public function unwrapErr(): never
         throw UnwrapException::unwrapErrOnOk($this->value);
     }
 
+    /**
+     * @return T
+     */
+    #[Override]
+    public function expect(string $message): mixed
+    {
+        return $this->value;
+    }
+
+    #[Override]
+    public function expectErr(string $message): never
+    {
+        throw UnwrapException::withMessage($message, $this->value);
+    }
+
     /**
      * @template U
      * @param U $default
diff --git a/src/Result.php b/src/Result.php
index c5e72a1..c10998f 100644
--- a/src/Result.php
+++ b/src/Result.php
@@ -74,6 +74,28 @@ public function unwrap(): mixed;
      */
     public function unwrapErr(): mixed;
 
+    /**
+     * 成功値を返します。失敗の場合は指定したメッセージで例外を投げます.
+     *
+     * @param string $message 失敗時の例外メッセージ（エラー値の要約が付加されます）
+     *
+     * @return ($this is Ok<mixed> ? T : never)
+     *
+     * @throws UnwrapException $this が Err の場合
+     */
+    public function expect(string $message): mixed;
+
+    /**
+     * エラー値を返します。成功の場合は指定したメッセージで例外を投げます.
+     *
+     * @param string $message 成功時の例外メッセージ（成功値の要約が付加されます）
+     *
+     * @return ($this is Err<mixed> ? E : never)
+     *
+     * @throws UnwrapException $this が Ok の場合
+     */
+    public function expectErr(string $message): mixed;
+
     /**
      * 成功値またはデフォルト値を返します.
      *
diff --git a/src/UnwrapException.php b/src/UnwrapException.php
index 6fb9eaf..ed8dca5 100644
--- a/src/UnwrapException.php
+++ b/src/UnwrapException.php
@@ -35,6 +35,14 @@ public static function unwrapErrOnOk(mixed $value): self
         return new self(\sprintf('called Result::unwrapErr() on an Ok value: %s', self::describe($value)));
     }
 
+    /**
+     * expect() / expectErr() 用に、呼び出し側のメッセージと値の要約から例外を生成します.
+     */
+    public static function withMessage(string $message, mixed $value): self
+    {
+        return new self(\sprintf('%s: %s', $message, self::describe($value)));
+    }
+
     /**
      * 例外メッセージ用に値の要約を生成します.
      *
diff --git a/tests/ErrTest.php b/tests/ErrTest.php
index 01b44af..97b1d31 100644
--- a/tests/ErrTest.php
+++ b/tests/ErrTest.php
@@ -59,6 +59,22 @@ public function unwrap_throws_exception(): void
         $err->unwrap();
     }
 
+    #[Test]
+    public function expect_throws_withGivenMessage(): void
+    {
+        $err = new Err('error');
+        $this->expectException(\LogicException::class);
+        $this->expectExceptionMessage('config file should be readable');
+        $err->expect('config file should be readable');
+    }
+
+    #[Test]
+    public function expectErr_returns_error_value(): void
+    {
+        $err = new Err('error');
+        $this->assertSame('error', $err->expectErr('should have an error'));
+    }
+
     #[Test]
     public function unwrap_throwsUnwrapException_withErrorValueInMessage(): void
     {
@@ -146,6 +162,15 @@ public function unwrap_withMultilineStringError_keepsMessageSingleLine(): void
         }
     }
 
+    #[Test]
+    public function expect_throwsUnwrapException_withErrorValueInMessage(): void
+    {
+        $err = new Err(new \RuntimeException('boom'));
+        $this->expectException(UnwrapException::class);
+        $this->expectExceptionMessage('config file should be readable: RuntimeException: boom');
+        $err->expect('config file should be readable');
+    }
+
     #[Test]
     public function unwrapErr_returns_error_value(): void
     {
diff --git a/tests/OkTest.php b/tests/OkTest.php
index 5235ab9..f8bf470 100644
--- a/tests/OkTest.php
+++ b/tests/OkTest.php
@@ -90,6 +90,22 @@ public function unwrapErr_throws_exception(): void
         $ok->unwrapErr();
     }
 
+    #[Test]
+    public function expect_returns_value(): void
+    {
+        $ok = new Ok(42);
+        $this->assertSame(42, $ok->expect('should have a value'));
+    }
+
+    #[Test]
+    public function expectErr_throws_withGivenMessage(): void
+    {
+        $ok = new Ok(42);
+        $this->expectException(\LogicException::class);
+        $this->expectExceptionMessage('should have an error');
+        $ok->expectErr('should have an error');
+    }
+
     #[Test]
     public function unwrapErr_throwsUnwrapException_withValueInMessage(): void
     {
@@ -114,6 +130,15 @@ public function __toString(): string
         $ok->unwrapErr();
     }
 
+    #[Test]
+    public function expectErr_throwsUnwrapException_withValueInMessage(): void
+    {
+        $ok = new Ok(42);
+        $this->expectException(UnwrapException::class);
+        $this->expectExceptionMessage('should have an error: 42');
+        $ok->expectErr('should have an error');
+    }
+
     #[Test]
     public function unwrapOr_returns_value(): void
     {
diff --git a/tests/Types/result.php b/tests/Types/result.php
index 1ba6366..5b7ec14 100644
--- a/tests/Types/result.php
+++ b/tests/Types/result.php
@@ -254,6 +254,17 @@ function testUnwrapOnGenericReceiver(Result $result): void
     assertType('RuntimeException', $result->unwrapErr());
 }
 
+/**
+ * expect / expectErr も unwrap / unwrapErr と同じ条件付き戻り値型が解決される.
+ *
+ * @param Result<int, RuntimeException> $result
+ */
+function testExpectOnGenericReceiver(Result $result): void
+{
+    assertType('int', $result->expect('should have a value'));
+    assertType('RuntimeException', $result->expectErr('should have an error'));
+}
+
 /**
  * 具象レシーバでの unwrapOr / unwrapOrElse: 実行時に起こり得ない側の型を混ぜない.
  *