  Binary black holes occupy a special place in our quest for understanding the
evolution of galaxies along cosmic history. If massive black holes grow at the
center of (pre-)galactic structures that experience a sequence of merger
episodes, then dual black holes form as inescapable outcome of galaxy assembly.
But, if the black holes reach coalescence, then they become the loudest sources
of gravitational waves ever in the universe. Nature seems to provide a pathway
for the formation of these exotic binaries, and a number of key questions need
to be addressed: How do massive black holes pair in a merger? Depending on the
properties of the underlying galaxies, do black holes always form a close
Keplerian binary? If a binary forms, does hardening proceed down to the domain
controlled by gravitational wave back reaction? What is the role played by gas
and/or stars in braking the black holes, and on which timescale does
coalescence occur? Can the black holes accrete on flight and shine during their
pathway to coalescence? N-Body/hydrodynamical codes have proven to be vital
tools for studying their evolution, and progress in this field is expected to
grow rapidly in the effort to describe, in full realism, the physics of stars
and gas around the black holes, starting from the cosmological large scale of a
merger. If detected in the new window provided by the upcoming gravitational
wave experiments, binary black holes will provide a deep view into the process
of hierarchical clustering which is at the heart of the current paradigm of
galaxy formation. They will also be exquisite probes for testing General
Relativity, as the theory of gravity. The waveforms emitted during the
inspiral, coalescence and ring-down phase carry in their shape the sign of a
dynamically evolving space-time and the proof of the existence of an horizon.
