The computer program AVSYS has been developed to analyze the availability of fusion power plants. A parametric study has been conducted on MARS. In order to bring up the availability to acceptable levels redundancy is needed in the neutral beam injection, ECRH, ICRH, direct convertor, and the central cell magnet coils (one coil/side redundant). At the same time, an improvement in quality, maintenance, design (hence failure rates and repair times) is needed for the magnets, as well as the neutral beam subsystem and the direct convertor.