  While the Pierre Auger Observatory is a very successful instrument for
ultra-high energy cosmic ray (UHECR) detection, it is increasingly used as part
of various types of multi-messenger searches, in which it contributes with
searches for air showers induced by atomic nuclei, neutrons, photons, and
neutrinos. We present an overview of the multi-messenger activities of the
Pierre Auger Observatory. The overview includes: searches for ultra-high energy
photons and neutrinos detected by the Pierre Auger Observatory in coincidence
with gravitational wave events detected by LIGO and Virgo; searches for
correlations of the arrival directions of UHECRs detected by the Pierre Auger
Observatory and high-energy neutrinos detected by IceCube and ANTARES; searches
for Galactic neutrons; the multi-messenger campaign "Deeper, Wider, Faster",
aiming for common observations of a variety of complementary instruments. We
discuss the methods and results of these searches.
