  Astrophysical space missions deliver invaluable information about our
universe, stellar dynamics of our galaxy, and motion of celestial bodies in the
solar system. Astrometric space missions SIM and Gaia will determine distances
to stars and cosmological objects as well as their physical characteristics and
positions on the celestial sphere with microarcsecond precision. These and
other space missions dedicated to exploration of the solar system are
invaluable for experimental testing of general relativity. Permanently growing
accuracy of space and ground-based astronomical observations require
corresponding development of relativistic theory of reference frames, motion of
celestial bodies, and propagation of light/radio signals from a source of
light/radio to observer. Such theory must be based on Einstein's general
relativity and account for various relativistic effects both in the solar
system and outside of its boundary. We describe a hierarchy of the relativistic
frames adopted by the International Astronomical Union in 2000, and outline
directions for its theoretical and practical extentions by matching the IAU
2000 reference frames in the solar system to the cosmological
Friedman-Robertson-Walker reference frame and to the frames used in the
parametrized post-Newtonian formalism.
