Multi-messenger Astrophysics with the Pierre Auger Observatory

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.