  Massive black hole binaries (MBHBs) are unavoidable outcomes of the
hierarchical structure formation process, and according to the theory of
general relativity are expected to be the loudest gravitational wave (GW)
sources in the Universe. In this article I provide a broad overview of MBHBs as
GW sources. After reviewing the basics of GW emission from binary systems and
of MBHB formation, evolution and dynamics, I describe in some details the
connection between binary properties and the emitted gravitational waveform.
Direct GW observations will provide an unprecedented wealth of information
about the physical nature and the astrophysical properties of these extreme
objects, allowing to reconstruct their cosmic history, dynamics and coupling
with their dense stellar and gas environment. In this context I describe
ongoing and future efforts to make a direct detection with space based
interferometry and pulsar timing arrays, highlighting the invaluable scientific
payouts of such enterprises.

Cosmology and Nongalactic Astrophysics