The status of the Simulation Project for the ATLAS experiment in 2008

We describe the components of the Simulation Project in the ATLAS experiment at the end of 2008. Starting from an ATLAS Software Overview, the available Generators are presented (hard scattering, cosmics, beam-gas, beam-halo, single particle). Detector description improvements have focused on commissioning layouts and new implementation of inert material. Core Simulation is reviewed and described with a focus on parameter optimization, physics list choice, 3D visualization, large-scale production, and validation procedures. Fast Simulation is also described and its performance is evaluated in comparison with the results of full simulation. Digitization, the last step of the simulation process, is described, including bytestream format, pileup problems, and data overlay.