Geant4  (for GEometry ANd Tracking) is a platform for "the simulation of the passage of particles through matter," using Monte Carlo methods. It is the successor of the GEANT series of software toolkits developed by CERN, and the first to use Object oriented programming (in C++). Its development, maintenance and user support are taken care by the international Geant4 Collaboration. Application areas include high energy physics and nuclear experiments, medical, accelerator and space physics studies. The software is used by a number of research projects around the world.
The Geant4 software and source code is freely available from the project web site; until version 8.1 (released June 30, 2006), no specific software license for its use existed; Geant4 is now provided under the Geant4 Software License.
Geant4 includes facilities for handling geometry, tracking, detector response, run management, visualization and user interface. For many physics simulations, this means less time need be spent on the low level details, and researchers can start immediately on the more important aspects of the simulation.
Following is a summary of each of the facilities listed above:
Geometry is an analysis of the physical layout of the experiment, including detectors, absorbers, etc., and considering how this layout will affect the path of particles in the experiment.
Geant4 can also optionally perform histogramming, but it requires external software that implements the AIDA framework in order to do so.
Some high energy physics experiments using Geant4
BaBar and GLAST at SLAC
Because of its general purpose nature, Geant4 is well suited for development of computational tools for analysing interactions of particle with matter in many areas. These include:
Space applications  where it is used to study interactions between the natural space radiation environment and space hardware or astronauts;
^ S. Agostinelli et al.: Geant4 - a simulation toolkit, Nuclear Instruments and Methods in Physics Research A 506 (2003) 250-303