Commissioning of the ATLAS reconstruction software with first data

Looking towards first LHC collisions, the ATLAS detector is being commissioned using all types of physics data available: cosmic rays, beam-halo and beam-gas events produced during LHC single beam operations. In addition to putting in place the trigger and data acquisition chains, commissioning of the full software chain is a primary goal. This is interesting not only to ensure that the reconstruction, monitoring and simulation chains are ready to deal with LHC physics data, but also to understand the detector performance in view of achieving the physics requirements. Cosmic rays have allowed us to study the ATLAS detector in terms of efficiencies, resolutions, channel integrity, and alignment and calibrations. They have also allowed us to test and optimize the muon combined performance algorithms.