Algorithms, performance, development of the ATLAS High-Level trigger

The ATLAS trigger system has been used for the online event selection for three years of LHC data-taking and is preparing for the next run. The trigger system consists of a hardware level-1 and a software high-level trigger (HLT) which is implemented in a region-of-interest based level-2 stage and a event filter operating after event building with offline-like software. During the past three years, the luminosity and pile-up (number of collisions per beam crossing) has increased significantly placing escalating demands on the rejection and timing performance. The HLT algorithms advanced during this period to maintain and even improve performance. Also discussed is the work towards the merging of the two HLT levels in to a single level HLT.