Dependency Injection is a design pattern used in software development to improve the modularity and testability of code. It allows a class to receive its dependencies from an external source rather than creating them internally. This means that the class can focus on its core functionality without being tightly coupled to specific implementations of its dependencies. By using Dependency Injection, developers can easily swap out different implementations of a dependency, making it simpler to test and maintain the code. This approach promotes better organization and separation of concerns, leading to more flexible and reusable components in software applications.