Geant4 is an open source software toolkit for simulating the passage of high energy particles into matter and interacting with it. It covers particle energy range from O(100 eV) to multi-TeV, and has some very low energy modules for microdosimetry down to O(eV) and phonon transport (O(mK)). Its object oriented design allowed it to accommodate such variety of physics cases. In this presentation, we will give an overview of the software and its history. We will present applications from various domains which exploit Geant4 : HEP and nuclear science of course, but also space, medical and homeland security. We will mention some perspectives too and in particular the challenges that the HL-LHC poses to Geant4.