The RADIOASTRON mission (Andreyanov et al. 1986) is designed to achieve angular resolutions as fine as 6 microarcsec and will be used to study radio sources with very high brightness temperature. It will form an orbiting radio interferometer between a satellite radio telescope 10 m in diameter and several large ground-based radio telescopes. The orbit of the satellite makes it possible to have baselines from several thousands to 80,000 km. The satellite will be equipped with four dual polarization receivers at 327, 1665, 4830, and 22235 MHz, with local oscillators phase-locked to a ground-based hydrogen maser frequency standard via an S-band microwave link. The IF signals from the receivers will be transmitted to the ground by an X-band high data rate link and recorded on magnetic tape in the VLBA format. Participation of large radio telescopes from many countries is envisaged and encouraged.