  Due to the huge interaction rates and the tough experimental environment of
pp collisions at a centre-of-mass energy sqrt(s)=14TeV and luminosities of up
to 10^34 cm^-2 s^-1, one of the experimental challenges at the LHC is the
triggering of interesting events. In the ATLAS experiment a three-level trigger
system is foreseen for this purpose. The first-level trigger is implemented in
custom hardware and has been designed to reduce the data rate from the initial
bunch-crossing rate of 40 MHz to around 75 kHz. Its event selection is based on
information from the calorimeters and dedicated muon detectors. This article
gives an overview over the full first-level trigger system including the
Calorimeter Trigger, the Muon Trigger and the Central Trigger Processor. In
addition, recent results are reported that have been obtained from test-beam
studies performed at CERN where the full first-level trigger chain was
established successfully for the first time and used to trigger the read-out of
up to nine ATLAS sub-detector systems.
