CMS Silicon Tracker upgrade for HL-LHC

LHC is expected to increase its luminosity above the original nominal value of $\mathrm{10^{34} cm^{-2}s^{-1}}$, eventually achieving an order of magnitude increase after major upgrades will be performed after 2020. This configuration of the machine is known as High Luminosity-LHC (HL-LHC). CMS needs a completely new tracking system to maintain adequate performance in the HL-LHC environment and to provide tracking information for the Level-1 trigger decision. The most relevant requirements and constraints are summarized, along with highlights from some of the R\&D activities.