ATLFAST 2.0 a fast simulation package for ATLAS

This paper documents the update of the ATLFAST program for fast detector simula tion and physics analysis.The algorithm is implememnted as an independent packa ge written in Fortran 77 but exists also in an OO/C++ implementation. It can be used for fast event-simulation including the most crucial detector aspects: jet recosntruction in the calorimeters, moemntum/energy smearing for leptons and pho tons, magnetic field effects and missing transverse energy. It provides, star ting from the list of particles in the event, a list of recosntructed jets, isolated leptons and photons and the expected missing transverse energy. Optio nally, the package provides also list of reconstructed charged tracks. The packa ge is completed by ATLFAST-B, a set of useful routines which simulate efficien cies/rejections for tagging of b-jets, c-jets, tau-jets. In most cases, the dete ctor-dependent parameters are tuned to what expected for the performance of the ATLAS detector from full simulation.