  A review is presented on the properties, origin and evolutionary links of hot
subluminous stars which are generally believed to be extreme Horizontal Branch
stars or closely related objects. Amongst the field stars a large fraction of
sdBs are found to reside in close binaries. The companions are predominantly
white dwarfs, or low mass main sequence stars. Systems with sufficiently
massive WD companions may qualify as SN Ia progenitors. Recently evidence has
been found that the masses of some unseen companions might exceed the
Chandrasekhar mass, hence they must be neutron stars or black holes. Even a
planet has recently been detected orbiting the pulsating sdB star V391 Peg.
Quite to the opposite, in globular clusters, only very few sdB binaries are
found indicating that the dominant sdB formation processes is different in a
dense environment. Binary population synthesis models identify three formation
channels, (i) stable Roche lobe overflow, (ii) one or two common envelope
ejection phases and (iii) the merger of two helium white dwarfs. The latter
channel may explain the properties of the He-enriched sdO stars because their
binary fraction is lower than that of the sdBs by a factor of ten or more.
Pulsating subluminous B (sdB) stars play an important role for asteroseismology
as this technique has already led to mass determinations for a handful of
stars. A unique hyper-velocity sdO star moving so fast that it is unbound to
the Galaxy has probably been ejected by the super-massive black hole in the
Galactic centre. (abridged)
