Abstract: The principal concepts for undersea tunnels are the mined tunnel, the immersed tube and the shield-driven tunnel. The conditions favoring or opposing these alternatives, for rail or highway service, as well as the limits of their application, are discussed. Floating tunnels are also considered. Some sites are especially suited to layouts using combinations of the primary tunnelling methods. Special sites or conditions may indicate a hybrid design utilizing both bridges and tunnels, joined by man-made islands. Examples of all of these concepts as presented.