  The observations and theory of the exciting new class of galactic black hole
X-ray transients is reviewed. Seven of these systems have measured mass
functions or mass estimates in excess of stable neutron stars, making them
excellent black hole candidates. Two of them have revealed ``superluminal"
radio jets. Study of the hard and soft radiation from these sources has given
tight constraints on the physics of the viscosity of the accretion disk and
promises firm proof that these systems contain black holes. This will allow us
to search for black holes of more moderate mass and apply the knowledge of
these systems to suspected supermassive black holes in AGN's. The most
plausible mechanism for triggering the outburst of black hole candidate X-ray
transients is the ionization thermal instability. The disk instability models
can give the deduced mass flow in quiescence, but not the X-ray spectrum.
Advection models that can account for the quiescent X-ray spectrum are
difficult to match with the non-steady state, quiescent Keplerian disks.
Self-irradiation of the disk in outburst may not lead to X-ray reprocessing as
the dominant source of optical light, but may play a role in the ``reflare."
The hard power-law spectrum and radio bursts may be non-thermal processes
driven by the flow of pair--rich plasma from the disk at early times and due to
the formation of a pair--rich plasma corona at late times. The repeated
outbursts in systems like GRO J0422+32 suggest some sort of clock, but it is
unlikely that it has anything to do with a simple X-ray heating of the
companion star. These systems typically have low mass secondaries and their
evolutionary origin is still mysterious.
