  The High Luminosity LHC (HL-LHC) will integrate 10 times more luminosity than
the LHC, posing significant challenges for radiation tolerance and event pileup
on detectors, especially for forward calorimetry, and hallmarks the issue for
future colliders. As part of its HL-LHC upgrade program, the CMS collaboration
is designing a High Granularity Calorimeter to replace the existing endcap
calorimeters. It features unprecedented transverse and longitudinal
segmentation for both electromagnetic (ECAL) and hadronic (HCAL) compartments.
This will facilitate particle-flow calorimetry, where the fine structure of
showers can be measured and used to enhance pileup rejection and particle
identification, whilst still achieving good energy resolution. The ECAL and a
large fraction of HCAL will be based on hexagonal silicon sensors of 0.5 to 1
cm$^2$ cell size, with the remainder of the HCAL based on highly-segmented
scintillators with SiPM readout. The intrinsic high-precision timing
capabilities of the silicon sensors will add an extra dimension to event
reconstruction, especially in terms of pileup rejection. An overview of the
HGCAL project is presented, covering motivation, engineering design, readout
and trigger concepts, and expected performance.
