Coral microbiology is an emerging field, driven largely by a desire to understand, and ultimately prevent, the worldwide destruction of coral reefs. The mucus layer, skeleton and tissues of healthy corals all contain large populations of eukaryotic algae, bacteria and archaea. These microorganisms confer benefits to their host by various mechanisms, including photosynthesis, nitrogen fixation, the provision of nutrients and infection prevention. Conversely, in conditions of environmental stress, certain microorganisms cause coral bleaching and other diseases. Recent research indicates that corals can develop resistance to specific pathogens and adapt to higher environmental temperatures. To explain these findings the coral probiotic hypothesis proposes the occurrence of a dynamic relationship between symbiotic microorganisms and corals that selects for the coral holobiont that is best suited for the prevailing environmental conditions. Generalization of the coral probiotic hypothesis has led us to propose the hologenome theory of evolution.