  The ATLAS experiment at the Large Hadron Collider (LHC) will face the
challenge of efficiently selecting interesting candidate events in $pp$
collisions at 14 TeV centre-of-mass energy, whilst rejecting the enormous
number of background events. Therefore it is equipped with a three level
trigger system. The first level is is hardware based and uses coarse
granularity calorimeter information and fast readout muon chambers. The second
and third level triggers, which are software based, will need to reduce the
first level trigger output rate of ~ 75 kHz to ~ 200 Hz written out to mass
storage. The progress in commissioning of this system will be reviewed in this
paper.
