The study reported here was undertaken to collate details and review information about introductions of marine fauna and flora to Great Britain (England, Scotland and Wales). The focus of this review is on nature conservation and the work will be used to formulate policy on marine introductions in relation to nature conservation interests. The study evaluates non-native macro fauna and flora, which have been introduced from outside their natural range and have become established in the wild in British waters. This report evaluates their origin, date and method of introduction, the reasons for their success, rate of spread, current distribution, actual and potential effects on native ecosystems and nature conservation as well as commercial interests, and methods of control. Micro-organisms (less than 20 m) are not considered.