The High Luminosity (HL-LHC) project aims to increase the
integrated luminosity of CERN's Large Hadron Collider (LHC) by an
order of magnitude compared to its initial design. This requires a
large increase in bunch intensity and beam brightness compared to
the first three LHC runs, and hence poses serious collective-effects
challenges, related in particular to electron cloud, instabilities
from beam-coupling impedance, and beam-beam effects. Here, we
present the associated constraints and the mitigation measures
proposed to achieve the baseline performance of the upgraded LHC
machine. We also discuss the interplay of these mitigation measures
with other aspects of the accelerator, such as optics, physical and
dynamic apertures, the collimation system, and crab
cavities. Additional potential sources of intensity limitations are
also briefly discussed.