Commissioning of the ATLAS Trigger Event Selection with Single Beam and Cosmic Rays

ATLAS is a particle physics experiment at the CERN Large Hadron Collider (LHC), designed to discover new physics beyond the Standard Model as well as for precise measurements of known particles and interactions. The first $p-p$ collisions are foreseen later in 2009. The short LHC single-beam run in 2008 provided a ``stress test'' of the trigger. ATLAS has since collected more than $300 imes10^6$ cosmic-ray events for detector alignment and calibration purposes. These running periods allowed the trigger to be tested in different running conditions. This proceedings focuses on the experience gained in running the trigger in various commissioning environments.