The ATLAS Simulation Software

We present the status of the ATLAS Simulation Project. Recent detector description improvements have focussed on commissioning layouts, implementation of inert material, and comparisons to the as-built detector. Core Simulation is reviewed with a focus on parameter optimizations, physics list choices, visualization, large-scale production, and validation. A fast simulation is also briefly described, and its performance is evaluated with respect to the full Simulation. Digitization, the last step of the Monte Carlo chain, is described, including developments in pile up and data overlay.