  Of the three main types of binaries detectable through ground-based
gravitational wave observations, black hole-neutron star (BHNS) mergers remain
the most elusive. While candidates BHNS exist in the triggers released during
the third observing run of the Advanced LIGO/Virgo collaboration, no detection
has been confirmed so far. As for binary neutron star systems, BHNS binaries
allow us to explore a wide range of physical processes, including the neutron
star equation of state, nucleosynthesis, stellar evolution, high-energy
astrophysics, and the expansion of the Universe. Here, we review some of the
main features of BHNS systems: the distinction between disrupting and
non-disrupting binaries, the types of outflows that BHNS mergers can produce,
and the information that can be extracted from the observation of their
gravitational wave and electromagnetic signals. We also emphasize that for the
most likely binary parameters, BHNS mergers seem less likely to power
electromagnetic signals than binary neutron star systems. Finally, we discuss
some of the issues that still limit our ability to model and interpret
electromagnetic signals from BHNS binaries.
