This document introduces dependency injection (DI), a design pattern that allows for flexible organization of classes. DI frameworks allow objects to be constructed and injected with their dependencies automatically based on configuration, avoiding hardcoded dependencies. Aspect oriented programming (AOP) allows cross-cutting concerns like logging or error handling to be defined separately from application code. Inversion of control (IOC) "flips" construction such that classes request fully initialized dependencies rather than constructing them directly. DI promotes loose coupling, testability, and flexibility in swapping implementations.