  Compact binaries have long been a paradigm for accretion theory. Much of our
present view of how accretion occurs comes directly from the comparison of
theory with observations of these sources. Since theory differs little for
other objects such as active galaxies, increasing efforts have recently gone
into searching for correspondences in observed behaviour. This chapter aims at
giving a concise summary of the field, with particular emphasis on new
developments since the previous edition of this book. These developments have
been significant. Much of the earlier literature implicitly assumed that
accreting binaries were fairly steady sources accreting most of the mass
entering their vicinity, often with main-sequence companions, and radiating the
resulting accretion luminosity in rough isotropy. We shall see that in reality
these assumptions fail for the majority of systems. Most are transient; mass
ejection in winds and jets is extremely common; a large (sometimes dominant)
fraction of even short-period systems have evolved companions whose structure
deviates significantly from the zero-age main sequence; and the radiation
pattern of many objects is significantly anisotropic. It is now possible to
give a complete characterization of the observed incidence of transient and
persistent sources in terms of the disc instability model and formation
constraints. X-ray populations in external galaxies, particularly the
ultraluminous sources, are revealing important new insights into accretion
processes and compact binary evolution.
