  Pressure, together with temperature and magnetic field, is an important
thermodynamical parameter in physics. Investigating the response of a compound
or of a material to pressure allows to elucidate ground states, investigate
their interplay and interactions and determine microscopic parameters. Pressure
tuning is used to establish phase diagrams, study phase transitions and
identify critical points. Muon spin rotation/relaxation (muSR) is now a
standard technique making increasing significant contribution in condensed
matter physics, material science research and other fields. In this review, we
will discuss specific requirements and challenges to perform muSR experiments
under pressure, introduce the high-pressure muon facility at the Paul Scherrer
Institute (PSI, Switzerland) and present selected results obtained by combining
the sensitivity of the muSR technique with pressure.
