Geant4 is a simulation toolkit designed for a variety of applications, including HEP, nuclear physics and astrophysics experiments, as well as bio-medical and space science studies. It exploits advanced software engineering techniques and Object Oriented technology.

GEANT4 provides functionalities for all the typical domains of simulation:  geometry, tracking, detector response, run, event and track management, Particle Data Group compliant particle management, visualisation and user interface, as well as a large variety of physics processes and models.  
It is also complemented by modules for specific domains, such as space science applications.

Geant4 has been developed and maintained by a world-wide collaboration of more than 100 scientists from major laboratories, national organisations and the European Space Agency.

The main features of Geant4 will be described and results from a variety of applications will be presented, together with comparisons with experimental data.