  At the Large Hadron Collider at CERN the proton bunches cross at a rate of
40MHz. At the Compact Muon Solenoid experiment the original collision rate is
reduced by a factor of O (1000) using a Level-1 hardware trigger. A subsequent
factor of O(1000) data reduction is obtained by a software-implemented High
Level Trigger (HLT) selection that is executed on a multi-processor farm. In
this review we present in detail prototype CMS HLT physics selection
algorithms, expected trigger rates and trigger performance in terms of both
physics efficiency and timing.
