Understanding whether or not corals have the flexibility to change their complement of symbionts to adapt to changing climate is an important goal in reef ecology studies. While host fidelity to a single clade of symbiont is the dominant pattern in present-day corals, current estimates of flexibility are unreliable, because few studies have measured it rigorously and with adequately sensitive genetic techniques. Furthermore, flexibility must be explored at the level of the subclade, because generalisations of physiological performance among clades of Symbiodinium are not valid. In addition, we should not necessarily expect to see shifts among symbionts without distinct and enduring changes in environmental conditions. The few biogeographical comparisons available suggest that when corals encounter a new environment they have the flexibility to acquire different symbionts. Flexibility in the acquisition of symbionts is common at the time of infection, which in most corals occurs at, or shortly after, settlement. Consequently, flexibility is likely to be a feature of the life history of all species that must reacquire symbionts in each new generation.