Design Patterns provide a fundamental foundation to building maintainable and scalable software. Understanding how the patterns work, why they provide a benefit, and when to use them helps to ensure that software is built from reusable object oriented components. This DZone Refcard provides a quick reference to the original 23 Gang of Four design patterns, as listed in the book Design Patterns: Elements of Reusable Object-Oriented Software. Each pattern includes class diagrams, explanation, usage information, and a real world example.