  The simulation software for the ATLAS Experiment at the Large Hadron Collider
is being used for large-scale production of events on the LHC Computing Grid.
This simulation requires many components, from the generators that simulate
particle collisions, through packages simulating the response of the various
detectors and triggers. All of these components come together under the ATLAS
simulation infrastructure. In this paper, that infrastructure is discussed,
including that supporting the detector description, interfacing the event
generation, and combining the GEANT4 simulation of the response of the
individual detectors. Also described are the tools allowing the software
validation, performance testing, and the validation of the simulated output
against known physics processes.
