To meet the harsh radiation and pile-up environment of HL-LHC, CMS will add a timing layer between the tracker and the calorimeters. The detector will be called MIP Timing Detector (MTD) and will have a target resolution of 30-40 ps at the beginning of data-taking, degrading to only 40-60 ps at the end of operations. The MTD will be comprised of two detectors: a Barrel Timing Layer (BTL) made of scintillating LYSO crystal bars coupled with SiPMs, and an Endcap Timing Layer (ETL) made of Low-Gain Avalanche Diodes (LGADs). Successful R&D campaigns have been carried out, proving that the required performance is achievable with the chosen technologies.