The Working Group "0ffshore Nuclear Power Plants" was established by the Commission of the European Communities in 1971. At that time it appeared that the option offered by creating new sites offshore should be explored. The present development of nuclear power has not, however, reached the stage where offs hore siting can be considered as a near or medium term proposition. The purpose of this report is to summarize the state of the art for the provision of floating or fixed structures, or man-made islands of the size needed for the construction of nuclear and other power stations. It describes the main factors which must be taken into account in the design and location of such islands and provides an indication of feasibility and cost for each design at the present time. It deals mainly with the civil engineering problems and not other major factors, such as the law of the sea, the rights of countries to locate nuclear establishments off their coast, their safety, security, the energy connection with the mainland, the marine ecology, the logistics, etc. ; many of these problems are directly related to the site and have major economic and political implications. By studying application examples the Working Group has occupied itself with these other factors.