  The CMS Collaboration is preparing to build replacement endcap calorimeters
for the HL-LHC era. The new high-granularity calorimeter (HGCAL) is, as the
name implies, a highly-granular sampling calorimeter with approximately six
million silicon sensor channels (approx. 1.1cm2 or 0.5cm2 cells) and about 250
thousand channels of scintillator tiles readout with on-tile silicon
photomultipliers. The calorimeter is designed to operate in the harsh radiation
environment at the HL-LHC, where the average number of interactions per bunch
crossing is expected to exceed 140. Besides measuring energy and position of
the energy deposits, the electronics is also designed to measure the time of
their arrival with a precision in the order of 50ps. This paper summarises the
reasoning and ideas behind the HGCAL, describes the current status of the
project, and highlights some of the challenges ahead.
