  The inspiral and merger of two orbiting black holes is among the most
promising sources for the first (hopefully imminent) direct detection of
gravitational waves (GWs), and measurements of these signals could provide a
wealth of information about astrophysics, fundamental physics and cosmology.
Detection and measurement require a theoretical description of the GW signals
from all possible black-hole-binary configurations, which can include
complicated precession effects due to the black-hole spins. Modelling the GW
signal from generic precessing binaries is therefore one of the most urgent
theoretical challenges facing GW astronomy. This article briefly reviews the
phenomenology of generic-binary dynamics and waveforms, and recent advances in
modelling them.
