  Black hole accretion flows can be divided into two broad classes: cold and
hot. Cold accretion flows, which consist of cool optically thick gas, are found
at relatively high mass accretion rates. Prominent examples are the standard
thin disk, which occurs at a fraction of the Eddington mass accretion rate, and
the slim disk at super-Eddington rates. These accretion flows are responsible
for luminous systems such as active galactic nuclei radiating at or close to
the Eddington luminosity and black hole X-ray binaries in the soft state. Hot
accretion flows, the topic of this review, are virially hot and optically thin.
They occur at lower mass accretion rates, and are described by models such as
the advection-dominated accretion flow and luminous hot accretion flow. Because
of energy advection, the radiative efficiency of these flows is in general
lower than that of a standard thin accretion disk. Moreover, the efficiency
decreases with decreasing mass accretion rate. Observations show that hot
accretion flows are associated with jets. In addition, theoretical arguments
suggest that hot flows should produce strong winds. Hot accretion flows are
believed to be present in low-luminosity active galactic nuclei and in black
hole X-ray binaries in the hard and quiescent states. The prototype is Sgr A*,
the ultra-low-luminosity supermassive black hole at our Galactic center. The
jet, wind and radiation from a supermassive black hole with a hot accretion
flow can interact with the external interstellar medium and modify the
evolution of the host galaxy. Details of this "maintenance-mode feedback"
could, in principle, be worked out through theoretical studies and numerical
simulations of hot accretion flows.
