1. Single Responsibility Principle (SRP): A class should have only one responsibility and all its methods and properties should be related to that responsibility. 2. Open/Closed Principle (OCP): Classes should be open for extension, but closed for modification. 3. Liskov Substitution Principle (LSP): Derived classes should be substitutable for their base classes. 4. Interface Segregation Principle (ISP): A client should not be forced to implement an interface that it doesn’t use. 5. Dependency Inversion Principle (DIP): High-level modules should not depend on low-level modules; both should depend on abstractions.