In the face of concern about the impact of global warming, particularly in relation to coral bleaching and the threat of thermal stress on tropical reefs, there has been reawakening of interest in marginal reefs. However, it is doubtful if there is much scope for extension of reefs further poleward. Many mid-ocean settings at which reefs are presently marginal represent the limit of land that would be suitable; for example, there are no islands further north of Bermuda or south of Lord Howe Island to which corals could extend. Similarly there is an absence of suitable habitat on many of the mainland coasts beyond the present limit. Where corals are able to colonize, they will be subject to a plethora of other stresses, pollution, sedimentation, and the effects of ocean acidification which are likely to be felt first in these higher latitude settings.