Apicomplexans are a large group of protists, including several species of major medical importance, and evolved from free-living photosynthetic ancestors. The discovery of corallicolids, photosymbiosis-derived coral-infecting parasites, provides a model system for studying the transition from phototrophy to parasitism. Here we review the endosymbiotic origins of apicomplexans and discuss the implications of corallicolid biology for understanding this major evolutionary transition.