Wednesday 7 October 2009 Speaker: John Baines (PPD, RAL) Title: Commissioning of the ATLAS High Level Trigger Abstract: The ATLAS High Level Trigger (HLT) was successfully commissioned in 2008 both by running offline on data collected during a short period of single beam running and subsequently online during an extended period of cosmic running. The trigger was used to assign the data to different streams and 216M cosmic events were recorded. This talk gives a short overview of the ATLAS Trigger and describes the commissioning of the High Level Trigger in 2008. The subsequent improvements made in preparation for running in 2009 are presented and the strategy for commissioning the HLT with colliding beams is discussed.