Status and performance of the ATLAS MUON spectrometer

The ATLAS experiment at the Large Hadron Collider at CERN is currently waiting to record first data from proton- proton collisions in 2009. During the last two years the MUON spectrometer was commissioned with cosmic rays while being assembled in the underground experimental cavern. In summer 2008, during the start-up phase of the LHC, the first events from single beams were recorded with the fully functional ATLAS detector. Subsequently the whole ATLAS detector was operated and commissioned with cosmic particles. The acquired data samples (of more than 100 million events, both with and without magnetic field) allowed a detailed study of the detector performance. In addition they were used to fine tune the spectrometer for an optimal operation with the first proton-proton collisions. Here the observed performance of the ATLAS MUON detection system is presented. Both the precision and the trigger chambers are studied with cosmic rays, as well as the level-1 muon trigger. First results from combined studies with other subdetectors are also presented.