S — Single Responsibility Principle (SRP) - A class should have only one reason to change O — Open/Closed Principle (OCP) - Software entities should be open for extension but closed for modification. L — Liskov Substitution Principle (LSP) - Objects of a subclass should be able to replace objects of the parent class without breaking the application. I — Interface Segregation Principle (ISP) - Clients should not be forced to depend on interfaces they do not use. D — Dependency Inversion Principle (DIP) - High-level modules should not depend on low-level modules. Both should depend on abstractions.