The Compact Muon Solenoid (CMS) detector is described, which operates at the Large Hadron Collider (LHC) at CERN. The CMS detector was designed to study proton-proton and lead-lead collisions at a center-of-mass energy of 14 TeV (5.5 TeV for lead-lead collisions) and at luminosities up to 10^34 cm^-2 s^-1 (10^27 cm^-2 s^-1 for lead-lead collisions). The core of the CMS detector includes a high-magnetic-field and large-bore superconducting solenoid surrounding an all-silicon pixel and strip tracker, a lead-tungsten scintillating-crystals electromagnetic calorimeter, and a brass-scintillator sampling hadron calorimeter. The iron yoke of the flux-return is instrumented with four stations of muon detectors covering most of the 4π solid angle. Forward sampling calorimeters extend the pseudo-rapidity coverage to high values (|η| ≤ 5), ensuring very good hermeticity. The overall dimensions of the CMS detector are 21.6 m in length, 14.6 m in diameter, and a total weight of 12500 tons.The Compact Muon Solenoid (CMS) detector is described, which operates at the Large Hadron Collider (LHC) at CERN. The CMS detector was designed to study proton-proton and lead-lead collisions at a center-of-mass energy of 14 TeV (5.5 TeV for lead-lead collisions) and at luminosities up to 10^34 cm^-2 s^-1 (10^27 cm^-2 s^-1 for lead-lead collisions). The core of the CMS detector includes a high-magnetic-field and large-bore superconducting solenoid surrounding an all-silicon pixel and strip tracker, a lead-tungsten scintillating-crystals electromagnetic calorimeter, and a brass-scintillator sampling hadron calorimeter. The iron yoke of the flux-return is instrumented with four stations of muon detectors covering most of the 4π solid angle. Forward sampling calorimeters extend the pseudo-rapidity coverage to high values (|η| ≤ 5), ensuring very good hermeticity. The overall dimensions of the CMS detector are 21.6 m in length, 14.6 m in diameter, and a total weight of 12500 tons.