ATLAS Trigger: design and commissioning

The ATLAS detector at CERN's Large Hadron Collider (LHC) will be exposed to proton-proton collisions from beams crossing at 40 MHz. A three-level trigger system was designed to select potentially interesting events and reduce the incoming rate to 100-200 Hz. The first trigger level (LVL1) is implemented in custom-built electronics, the second and third trigger levels are realised in software. The trigger system and its design parameters will be described with focus on computing and data aquision challenges. The results from both commissioning cosmic runs and first experiences from the LHC beam in 2008 will be overviewed. These running periods allowed us to exercise the trigger system online, including its configuration and monitoring infrastructure, as well as reconstruction and selection algorithms. The details on the plans for commissioning the ATLAS trigger when the LHC starts operations will be presented.