In this talk, we will present the Geant4 software toolkit for the
Monte Carlo simulation of the passage of particles through
matter. This toolkit is used in particular, but not only, to simulate
the High Energy Physics detectors response. It consists of several
components, playing particular roles, like the geometry description
classes, needed to 'build' the model of the simulated devices,
tracking functionality allowing to 'propagate' the particles through
that geometry and physics models, electromagnetic and hadronic, to
simulate particular processes.  We will first briefly discuss all
those different parts of the toolkit, trying to illustrate how a
complete simulation is implemented. In the second part of the talk, we
will present recent developments in some selected areas including the
electromagnetic and hadronic physics models.