Dependency injection (DI) is a key feature of ASP.NET Core that simplifies application development and improves testability and maintainability. DI allows you to inject dependencies into classes or components rather than creating them directly. This promotes loose coupling between components and enables easier unit testing, better code organization, and flexibility in swapping out implementations. ASP.NET Core has built-in DI container support, but you can also use third-party containers if desired.