Geant4 is an open source radiation simulation toolkit widely used in particle and nuclear physics, space and medicine. It tracks radiation as particles through matter using Monte Carlo techniques. It exploits object-oriented programming technology in C++. Its strength is its capability to model very complex geometries and in the wealth of physics models that are continually under development and improvement. John is a founder member, involved in development since 1996. He will give some background, an outline of Geant4’s architecture, its current capabilities and the outlook for the future.