The three classical types of symbiotic relationships, i.e., those species living in close, heterospecific associations—commensalism, mutualism, and parasitism—are examined from coral reef environments in the Caribbean (Puerto Rico), eastern Pacific, and southeastern Florida. Recent studies show that many formerly regarded commensal relationships, where one partner benefits and the other is unaffected, are more accurately classified as examples of mutualism or parasitism. Both symbiotic partners benefit in mutualistic relationships, often revealing exceptional coevolutionary adaptations. Exemplary cases are (a) coral crustacean guards, (b) shrimp and fish cleaner-client relationships, and (c) damselfish-algal lawn interactions. Parasitism, wherein one partner benefits at the expense of another, has not received sufficient attention in eastern Pacific coral reef studies. Parasites, pathogens, and diseases are interdependent, and their effects or impacts largely depend on abiotic and biotic environmental stressors. The cause(s) of coral bleaching and mortality were vigorously investigated in the early 1980s, following the first reported worldwide reef disturbance. Diseases affecting corals (white syndrome, “trematodiasis”) and sea urchins (a ciliate protozoan) are of concern relative to eastern Pacific coral reefs. Lithophagine date mussels, long regarded as parasites of corals, benefit their hosts by providing a nitrogen source that increases growth and may best be regarded as mutualists.