  Ultraluminous X-ray sources (ULXs) represent a class of binary systems that
are more luminous than any black hole in our Galaxy. The nature of these
objects remained unclear for a long time. The most popular models for the ULXs
involve either intermediate mass black holes (IMBHs) or stellar-mass black
holes accreting at super-Eddington rates. In the last few years our
understating of these objects was significantly improved, which made the model
of super-Eddington accretion much preferable. Both the X-ray and optical
spectra provide evidence for the strong outflows coming from supercritical
accretion disk. Another surprising result was discovery of pulsations in four
ULXs, which claims that these systems must host neutron stars. Besides the
presence of pulsations, there is no sharp difference between ultraluminous
pulsars and normal ULXs. This fact implies that significant number of known
ULXs might eventually be neutron stars.
