Proton-proton collisions at the LHC design luminosity will produce an average of 25 events at the beam crossing frequency of 25 nsec. This input rate of 10^9 interactions every second must be reduced by a factor of at least 10^7 to 100 Hz, the maximum rate that can be archived by the online computer farm. In CMS this reduction is performed in two steps. At the first level all data is stored in pipelines after which no more than 100 kHz of the stored events are forwarded to the higher level trigger system. The first level trigger is based on the identification of muons, electrons, photons, jets and missing transverse energy. The interesting reactions, like for example the Higgs production, can be filtereded from the huge background of minimum bias reactions by requiring the presence of one or two high transverse momentum leptons.

The CMS detector is a general purpose detector specifically optimized for muon measurements which are performed by Drift Tubes (DT) located outside of the magnet coil in the barrel region and Cathode Strips Chambers (CSC) in the forward region. The CMS Muon System is also equipped with Resistive Plate Chambers (RPC) dedicated for triggering. The first level muon trigger uses all three kinds of muon detectors. The excellent spatial precision of DT and CSC ensures sharp momentum thresholds. Their multilayer structure provides a possibility of effective background rejection.The RPC superior time resolution ensures unambiguous bunch crossing identification. High granularity makes possible to work in high rate environment. Complementary features of muon chambers (DT/CSC) and dedicated trigger detectors (RPC) allows to build two trigger subsystems which deliver independent informations about detected particles, providing high efficiency and powerful background rejection.