Dependency Injection (DI) is a design pattern built into ASP.NET Core that allows objects to receive the services they depend on instead of creating those services themselves. The ASP.NET Core dependency injection container automatically manages the creation and lifetime of services and injects them wherever they are needed. For example, a controller can receive a database service, logging service, or email service through its constructor rather than instantiating those objects manually. Dependency Injection improves application architecture by promoting loose coupling, making components easier to test, maintain, and replace. It also supports the SOLID principles, especially the Dependency Inversion Principle, enabling developers to write cleaner and more modular code. ASP.NET Core supports three service lifetimes: Transient – A new instance is created every time the service is requested. Scoped – One instance is created per HTTP request. Singleton – A single instance is shared throughout the application's lifetime. By using Dependency Injection, developers can build scalable, maintainable, and testable enterprise applications while reducing code complexity and improving overall software quality.