Conventional views have implied that scleractinian corals were hermatypic and thus were potentially reef building from the onset of their appearance in early Mesozoic time in shallow-shelf carbonate environments. However, for almost 25 m.y. they held rather subordinate positions and generally did not participate in extensive ecologic reef framework. Evidence from both North America and Europe demonstrates that Triassic corals had ecologic preferences quite different from those in the Cenozoic. A key step in the evolutionary success of these corals may have been the advent of a symbiotic association with algae. This association, believed to have developed at the end of the Triassic, could have been responsible for thrusting scleractinians into prominent positions as reef builders and may account for their dramatic radiation in Mesozoic time.