The framework of General Relativity is applied to the problem of reduction of high-precision astrometric observations of the order of 1-mu-arcsec. Such precision is expected to be attained in the not so distant future by means of the space optical interferometers orbiting the Earth. Theoretical methods are described enabling one to construct astrometric reference systems involving the barycentric system, geocentric system, and satellite (observer) system. The relativistic transformations between the employed reference systems have been derived. The equations of geometric optics for the nonstationary gravitational field of the Solar system have been deduced. To solve these equations one can apply the techniques of asymptotic matching and approximation of functions. Integration of the equations of geometric optics results in the isotropic geodesic line connecting the source of emission (a star, a quasar) and an observer. This permits us to calculate the effects of relativistic deflection of the light due to monopole and quadrupole components of the gravitational field of the Sun and the planets taking into account their motions and rotations. Transformations between the reference systems are used to calculate the light aberration occurring when passing from the satellite system to the geocentric system and from the geocentric system to the barycentric system. Aberrational corrections are applied to transform the observed position of a celestial object to the barycentric system with properly taking into account the relativistic light deflection. The barycentric components of the observed position vector, reduced in such a manner, are corrected for parallax and proper motion of a celestial object using the classical techniques of Euclidean geometry.