Dependency Injection is a design pattern used in software development to improve code modularity and testability. Instead of a class creating its own dependencies, these dependencies are provided to the class from an external source. This allows for easier management of dependencies and promotes the use of interfaces. By using Dependency Injection, developers can swap out implementations without changing the class that uses them. This makes it simpler to test components in isolation, as mock or stub versions of dependencies can be injected during testing. Overall, it leads to cleaner, more maintainable code.