The Object Oriented (OO) approach uses Unified Modeling Language (UML) diagrams for documenting the analysis and design stage, and languages supporting the OO paradigms (C++, Java, C#, etc.) for the programming stage. Some UML tools can generate more or less complete C++ code from design specifications. This chapter presents the UML graphical notations to illustrate OO items. It presents some of the main paradigms encountered within OO analysis and design, class, object, inheritance, encapsulation, polymorphism and genericity. The main goal of the OO approach (analysis or design) is to capture concepts and entities of the studied system in order to associate classes to each concept or entity of the studied system. UML diagrams are the documentation of OO analysis or design. The two two main used diagrams for GranOO documentation, class diagram and sequence diagram, are also discussed.