Geant4 - a simulation toolkit

Geant4 is a set of C++ libraries using the Object Oriented Programming paradigm to simulate the interactions of particles as they pass through matter. It has been successfully used in LHC experiments, in many other particle physics experiments, in nuclear physics, in space and in medicine. It has a long history and heritage and continues to be developed and extended. I will describe it from the point of view of a developer who has been with the project since its inception, and describe its features — including multithreading, event biasing, reverse MonteCarlo, parallel worlds — and some of it uses.