SOLID = 5 OO design principles. S (Single Responsibility): a class/module has one responsibility (one job) and should change for one reason. O (Open/Closed): extend without modifying existing code. L (Liskov Substitution): subtypes are safely substitutable (keep the contract). I (Interface Segregation): small focused interfaces, no unused methods. D (Dependency Inversion): depend on abstractions; inject implementations (DI).