ATLAS trigger for first physics and beyond

ATLAS is a multi-purpose spectrometer built to perform precision measurements of Standard Model parameters and is aiming at discovery of Higgs particle, Super Symmetry and possible other physics channels beyond Standard Model. Operating at 14 TeV center of mass energy ATLAS will see 40 million events per second at nominal luminosity with about 25 overlapping interactions. Most of the events are inelastic proton-proton interactions with only few W, Z bosons or ttbar pairs produced each second, and expectations for Higgs or SUSY production cross-section are much smaller than that. ATLAS trigger has a difficult task to select one out of $10^5$ events online and to ensure that most physics channels of interests are preserved for analysis. In this talk we will review the design of ATLAS trigger system, the trigger menu prepared for initial LHC run as well as for high luminosity run. The expected trigger performance of the base-line ATLAS physics programs will be reviewed and first results from the commissioning period will be given. The methods to measure trigger efficiencies and biases directly from data will be discussed.