With the detection of gravitational waves emitted during black hole and neutron star mergers, LIGO has recently opened the field of gravitational wave astrophysics. In this talk I will review the astrophysical processes that may be responsible for the formation of the observed events. The event rate distribution with mass, spins, eccentricity and redshift may be used to discriminate among different processes that lead to black hole mergers. I will show that the standard astrophysical merger channels are already in tension with LIGO/VIRGO observations. New ideas may be needed to explain the origin of the detected sources. I will discuss the possibility that black hole mergers happen in active galactic nuclei where the interaction with a gaseous disk helps to form binaries, and a combination of dynamical and gas effects facilitate the merger of the binaries.