Marginal basins are common features of present-day plate tectonics. Whereas some may represent trapped segments of normal ocean floor, many owe their origin to extensional seafloor spreading behind active volcanic arcs. They exhibit a variety of forms. Some are completely intraoceanic; others develop at continental margins, where back-arc spreading may lead to the detachment and dispersal of continental fragments. Marginal basins can be recognized in the early stages of formation; others have developed through more than one pulse of back-arc extension, and some have aborted shortly after formation. Closure of marginal basins may result in preservation of part of the basin floor as obducted ophiolite. Although the reasons why seafloor spreading occurs behind volcanic arcs are still imperfectly understood, all suggested mechanisms invoke a strong link with subduction. Thus if subduction occurred in the past it is logical to expect that fossil marginal basins may be preserved in the geological record. However, allowing for the gradually evolving thermal and chemical nature of the Earth's mantle, ancient marginal basins need not necessarily duplicate every feature of modern ones. This contribution examines possible Phanerozoic, Proterozoic and Archaean marginal basin analogues in the light of the geological features shown by modern basins and attempts to assess their importance for crustal development.