  With the high bunch-crossing and interaction rates and potentially large
event sizes the experiments at the LHC challenge data acquisition and trigger
systems. Within the ATLAS experiment, a multi-level trigger system based on
hardware and software is employed to cope with the task of event-rate
reduction. This review article gives an overview of the trigger of the ATLAS
experiment highlighting the design principles and the implementation of the
system and provides references to more detailed information. In addition, first
trigger- performance studies and an outlook on the ATLAS event-selection
strategy are presented.
