  Recent modern space missions deliver invaluable information about origin of
our universe, physical processes in the vicinity of black holes and other
exotic astrophysical objects, stellar dynamics of our galaxy, etc. On the other
hand, space astrometric missions make it possible to determine with
unparalleled precision distances to stars and cosmological objects as well as
their physical characteristics and positions on the celestial sphere.
Permanently growing accuracy of space astronomical observations and the urgent
need for adequate data processing algorithms require corresponding development
of an adequate theory of reference frames along with unambiguous description of
propagation of light rays from a source of light to observer. Such a theory
must be based on the Einstein's general relativity and account for numerous
relativistic effects both in the solar system and outside of its boundary. The
main features of the relativistic theory of reference frames are presented in
this work. A hierarchy of the frames is described starting from the perturbed
cosmological Friedmann-Robertson-Walker metric and going to the observer's
frame through the intermediate barycentric and geocentric frames in the solar
system. Microarcsecond astrometry and effects of propagation of light rays in
time-dependent gravitational fields are discussed as well.
