The CMS trigger system is designed to select events of potential physics interest from a high interaction rate of proton-proton (and heavy ion) collisions at the LHC. It consists of two levels: the first level (L1), implemented in hardware, selects events based on detector signals consistent with electrons, photons, muons, tau leptons, jets, or missing transverse energy. The second level (HLT), implemented in software, further refines the output stream to select an average rate of 400 Hz for offline storage. The trigger thresholds are adjusted based on LHC instantaneous luminosity to maintain an output rate of 100 kHz. The L1 trigger includes a calorimeter trigger system (RCT and GCT) and a muon trigger system (DT, CSC, RPC). The RCT processes energy deposits from ECAL and HCAL, while the GCT sorts and calculates global quantities like missing transverse energy. The muon trigger system includes local track segments from DT, CSC, and RPC, which are processed to identify muon candidates. The GMT merges muon candidates from different systems to provide final trigger candidates. The trigger system also handles heavy ion collisions, with different conditions for PbPb and pPb runs. The CMS trigger system was tested during LHC Run 1, selecting interesting events at high luminosities. The system's performance was evaluated in various physics analyses, including Higgs boson, top quark, supersymmetry, and B physics. The trigger menus, including L1 and HLT, were designed to optimize event selection. The system's operation and evolution during LHC Run 1 were described, with a focus on data collection, monitoring, and performance. The CMS trigger system is essential for selecting interesting events for offline analysis, ensuring efficient data processing and physics studies.The CMS trigger system is designed to select events of potential physics interest from a high interaction rate of proton-proton (and heavy ion) collisions at the LHC. It consists of two levels: the first level (L1), implemented in hardware, selects events based on detector signals consistent with electrons, photons, muons, tau leptons, jets, or missing transverse energy. The second level (HLT), implemented in software, further refines the output stream to select an average rate of 400 Hz for offline storage. The trigger thresholds are adjusted based on LHC instantaneous luminosity to maintain an output rate of 100 kHz. The L1 trigger includes a calorimeter trigger system (RCT and GCT) and a muon trigger system (DT, CSC, RPC). The RCT processes energy deposits from ECAL and HCAL, while the GCT sorts and calculates global quantities like missing transverse energy. The muon trigger system includes local track segments from DT, CSC, and RPC, which are processed to identify muon candidates. The GMT merges muon candidates from different systems to provide final trigger candidates. The trigger system also handles heavy ion collisions, with different conditions for PbPb and pPb runs. The CMS trigger system was tested during LHC Run 1, selecting interesting events at high luminosities. The system's performance was evaluated in various physics analyses, including Higgs boson, top quark, supersymmetry, and B physics. The trigger menus, including L1 and HLT, were designed to optimize event selection. The system's operation and evolution during LHC Run 1 were described, with a focus on data collection, monitoring, and performance. The CMS trigger system is essential for selecting interesting events for offline analysis, ensuring efficient data processing and physics studies.