S.O.L.I.D is an acronym that represents five principles of object-oriented programming and code design. It was theorized by our beloved Uncle Bob (Robert C. Martin) by the year 2000. The author Michael Feathers was responsible for creating the acronym: [S]ingle Responsibility Principle [O]pen/Closed Principle [L]iskov Substitution Principle [I]nterface Segregation Principle [D]ependency Inversion Principle Single Responsibility Principle (SRP) - A class should have one, and only one, reason to change. Open/Closed Principle (OCP) - You should be able to extend a classes behavior, without modifying it. Liskov Substitution Principle (LSP) - Derived classes must be substitutable for their base classes. Interface Segregation Principle (ISP) - Make fine grained interfaces that are client specific. Dependency Inversion Principle (DIP) - Depend on abstractions, not on concretions.