Commissioning of the ATLAS High Level Muon Trigger with Beam Collisions

The ATLAS experiment at CERN's Large Hadron Collider (LHC) has taken its first data with colliding beams. The LHC targets to deliver an integrated luminosity 1-fb in the run period 2010/2011 at luminosities of up to 10^32 cm^-2 s^-1, which requires active rejection of events in the trigger systems. The muon system is the largest sub-detector of the ATLAS experiment and has the capability to reconstruct muons in standalone mode, as well as in combination with the Inner Detector tracking systems. The High Level Muon Trigger (HLT) is split into two levels, each with standalone and combined algorithms. The performance of these algorithms relative to muons reconstructed by the offline software in pp collision data is presented.