Following detection by advance LIGO, gravitational wave (GW) stocks are on the rise. The low frequency GW Universe is likely dominated by signals emitted by a cosmological population of massive black hole binaries (MBHBs). I will review several aspect of MBH astrophysics, including their formation, evolution, interaction with their environment and GW emission. I will then discuss prospect of GW detection with pulsar timing arrays and/or future space based interferometers such as the laser interferometer space antenna (LISA) and for pulsar timing arrays.