1. SRP The Single Responsibility Principle: -- a class should have one, and only one, reason to change. 2. OCP The Open Closed Principle: -- you should be able to extend a class's behavior, without modifying it. 3. LSP The Liskov Substitution Principle: -- derived classes must be substitutable for their base classes. 4. ISP The Interface Segregation Principle: -- make fine grained interfaces that are client specific. 5. DIP The Dependency Inversion Principle -- depend on abstractions not on concrete implementations.