The CMS trigger system is designed to select events of potential physics interest from the high interaction rate of proton-proton and heavy ion collisions at the LHC. The system consists of two levels: the first level (L1) is implemented in hardware and selects events containing candidate objects such as electrons, photons, muons, tau leptons, jets, or missing transverse energy. The second level (HLT) is implemented in software and further refines the output stream. The L1 trigger thresholds are adjusted to restrict the output rate to 100 kHz, while the HLT selects an average rate of 400 Hz for offline storage. The paper describes the objectives, strategy, and performance of the trigger system during Run 1 of the LHC, including details on the L1 calorimeter and muon trigger systems, the global trigger system, and the high-level trigger system. The trigger system is crucial for selecting interesting events for offline analysis, with the L1 trigger making a final decision within 4 μs after a collision. The HLT further refines the purity of the physics objects, ensuring efficient data storage and analysis.The CMS trigger system is designed to select events of potential physics interest from the high interaction rate of proton-proton and heavy ion collisions at the LHC. The system consists of two levels: the first level (L1) is implemented in hardware and selects events containing candidate objects such as electrons, photons, muons, tau leptons, jets, or missing transverse energy. The second level (HLT) is implemented in software and further refines the output stream. The L1 trigger thresholds are adjusted to restrict the output rate to 100 kHz, while the HLT selects an average rate of 400 Hz for offline storage. The paper describes the objectives, strategy, and performance of the trigger system during Run 1 of the LHC, including details on the L1 calorimeter and muon trigger systems, the global trigger system, and the high-level trigger system. The trigger system is crucial for selecting interesting events for offline analysis, with the L1 trigger making a final decision within 4 μs after a collision. The HLT further refines the purity of the physics objects, ensuring efficient data storage and analysis.