The high-luminosity upgrade of the Large Hadron Collider (HL-LHC) will provide a unique opportunity to extend the physics reach of the CMS experiment by achieving unprecedented precision in the study of Standard Model (SM) processes and by increasing its discovery potential for physics beyond the SM to higher energy scales and rarer processes. The HL-LHC, however, will also bring experimental and instrumental challenges with its harsh environment: high radiation levels, high number of particle collisions per bunch crossing (pileup of 140-200), and extreme final state particle multiplicities. The CMS Collaboration has embarked on an ambitious upgrade program to improve the spatial granularity and timing resolution, to extend the coverage of precision detectors to higher absolute pseudorapidities ($\eta$), to increase the data rate that can be recorded, while using more robust and more radiation hard technologies.

In this paper, the main features of the CMS Phase-2 upgrade are reviewed. New high-granularity detectors will be installed: a silicon pixel and strip tracker with larger coverage ($|\eta|<4$), an “imaging” endcap calorimeter ($1.5<|\eta|<3$), an extended muon system in the forward region ($|\eta|<2.8$) including Gas Electron Multiplier detectors. The timing precision will also be significantly enhanced by dedicated timing detectors with 30-50 ps resolution ($|\eta|<3$), supplemented by improved timing information from muon detectors and calorimeters with upgraded electronics. Fully reconstructed tracks with transverse momentum of $p_\mathrm{T} > 2$~GeV and particle-flow at the level-1 hardware trigger will enable more efficient online event selection, with an increased level-1 rate (750 kHz) and latency (12.5 $\mu$s). Trigger objects will be available at 40 MHz for monitoring and restricted data analysis through a Scouting System. The high-level trigger with a heterogeneous architecture will provide a 7.5 kHz output rate to benefit from the increased luminosity.}