Single Responsibility, Open-Closed, Liskov Substitution, Interface Segregation, Dependency Inversion - five principles for writing maintainable object-oriented code.

SRP: a class has one reason to change. OCP: open for extension, closed for modification. LSP: subtypes must be substitutable for their base types. ISP: prefer many small interfaces over one large one. DIP: depend on abstractions, not concretions. Violations of these principles typically manifest as tight coupling, hard-to-test code, and classes that break when unrelated features change.