The project of an Optical Monitor (OM) for X-ray satellites, in particular the JET-X (Joint European Telescope for X-ray astronomy) experiment (Wells et al., 1991), derives from the scientific need of having complete data coverage at various wavelengths, UV and optical, of the observed X-ray sources, because these data are essential for a deeper understanding of the various classes of objects. When studying variable sources and/or transient astronomical phenomena, one needs that the multifrequency observations be performed essentially at the same time, because it is the knowledge of the simultaneous optical and X-ray behaviour of a source which contributes substantially to the clarification of its nature. In principle optical observations simultaneous with X-ray ones can be performed from ground based telescopes. However the complexity of satisfying the constraints typical of the optical telescopes (weather conditions, source observability) and of the X-ray instrumentation (e.g. orbital constraints) lead inevitably to a substantial loss of observing time. Therefore the only practical way of having an optimal utilization of the time available for X-ray observations, together with the wealth of scientific potential of simultaneous UV-optical observations, is to have a small telescope to be part of the same space mission.