LHCb Trigger System
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The LHCb detector is designed to study with high precision rare decays of and CP 
asymmetries in b-flavoured hadrons produced at the LHC. Though the LHC luminosity 
is large, the b-production cross section is less than 1% of the total  inelastic 
cross section. Also the branching ratios of typical b-decays of interest are rather 
small, often below 10^-4. The LHCb trigger is of crucial importance in selecting 
among the bulk of collisions those that are of interest for b-physics studies. It 
is based on a 2-level system. The first level, Level-0, is implemented in hardware 
and uses information from the calorimeter, muon and pile-up systems to select events 
containing particles with relatively large transverse momentum, typically above 1-2 
GeV. The Level-0 trigger accepts events at a rate of 1 MHz. All the detector 
information is then read out and fed into the High Level Trigger. This software 
trigger runs in the event filter farm composed of about 2000 CPUs. Algorithms 
separated in dedicated trigger streams exploit the Level-0 triggering information 
and refine the selection using both the tracks high transverse momenta and large 
impact parameters characteristic of b-decays.  Events are selected at a rate of 
2kHz and sent for mass storage and subsequent offline reconstruction and analysis. 
The current status and expected performance of the trigger system are described.