The GranOO API is composed of four libraries: sandbox, util, geometrical and Discrete Element Method (DEM). This chapter details the GranOO API. The libGeometrical is the core of the GranOO workbench because a DEM computation involves massive geometrical calculations. The DEM library provides classes that model the various concepts used in the discrete element method. The libMySandbox function is to receive custom developments. The libGeometrical allows intuitive mathematical writing and high-level processing such as changing frame operations. The libDEM is dedicated to the various concepts used in the discrete element method. The libMySandbox allows us to share these developments with all the projects developed by a user. The generic SetOf class implements the mathematical set concept with additional features. In conclusion, the GranOO API provides a coherent extensible framework in which the user focuses on the physical description of its numerical experiments and delegates the technical aspects to the API.