OpenGeoSys (OGS) is a scientific open source project for the development of numerical methods for the simulation of thermo-hydro-mechanical-chemical (THMC) processes in porous and fractured media. OGS is implemented in C++, it is object-oriented with an focus on the numerical solution of coupled multi-field problems (multi-physics). Parallel versions of OGS are available relying on both MPI and OpenMP concepts. Application areas of OGS are currently CO2 sequestration, geothermal energy, water resources management, hydrology and waste deposition. OGS is comprised of the THMC-simulator (simply referred to as OGS) and a visualization tool (Data Explorer). OGS is developed by the OpenGeoSys Community. It has to be noticed that the current OGS-6 version is under very heavy development and vivid discussion, and does not implement all functionality from OGS-5.