Geant4 is a software toolkit, written in C ++, for simulating the tracking of particles in the matter with the Monte Carlo approach. The software is developed and maintained by an International Collaboration of scientists belonging to different institutions, including INFN. Geant4 can be used for applications in high energy physics, astrophysics, medical physics, particle astrophysics and nuclear physics. The software provides all the tools needed to a complete Monte Carlo simulation of the experimental set-up, including modeling of geometry, detector response, event management and user interface. In Geant4 there is a wide range of physical models capable of describing interactions of particles with matter; there are several alternative models, which can be chosen by the user for many physical processes. Geant4 source code and libraries are freely available, along with manuals, from the Geant4 home page.