The decorator design pattern allows you to dynamically add behavior to your classes without modifying the original class. This allows you to layer in new functionality to a class while keeping different concerns cleanly separated. The decorator pattern is especially useful for when you need to apply cross cutting concerns like logging, authorization, and caching, but can also be used to modify data passed to and from an object. Understanding how to apply the decorator pattern gives you a powerful tool when you need to add behavior to and extend the functionality of your classes.