Commissioning the ATLAS detector with single beam and cosmic data

During 2008 the ATLAS experiment went through an intense period of preparation to have the detector fully commissioned for the first beam period. System-by-system commissioning was interspaced by so-called milestone weeks, during which the full system was run under conditions as close as possible to collision data taking. This period culminated in full 24/7 operation starting about 2 weeks before the first beam traversed ATLAS. In the about 30 hours of beam time available to ATLAS in 2008, the systems went through a rapid setup sequence, from successfully recording the first ever bunch reaching ATLAS, to setting up the timing of the trigger system synchronous to the incoming single beams. So-called splash events were recorded, were the beam is stopped on a collimator 140 m upstream of ATLAS, showering the experiment with millions of particles per beam shot. These events were found to be extremely useful for timing setups. After the stop of beam operation for 2008 the experiment went through an extensive cosmic ray data taking campaign, resulting in more than 200 million cosmic ray events recorded. These events have since been used to make significant progress on the calibration and alignment of the detector. The talk will describe the commissioning programme leading up to the first beam, the events around the first ever beam seen in ATLAS, and describe the results obtained from both the single beam data and the cosmic data reco rded since.