Coral reefs are critical habitats that support an abundance of marine life while also being economically important to millions of people that rely on reef-based industries such as tourism and fishing to sustain local communities. However, reef ecosystems globally are degrading at alarming rates due to anthropogenic impacts including ocean warming, poor water quality and over exploitation of marine stocks. Contributing to the decline in reef health has been coral disease outbreaks that can change benthic community assemblages, influence trophic networks, and impact the broad ecosystem services provided by reefs. Coral diseases have been investigated for over 50 years and cases of disease reported from almost every reef ecosystem on the planet. Disease is a natural component of the reef ecosystem, though increased disturbances from anthropogenic impacts have driven unprecedented outbreaks with some coral species in the Caribbean region now listed on the endangered species register. An extensive array of coral diseases has been described (> 40), though many have poor macroscopic and diagnostic descriptions with little associated epizootic and etiologic information. This chapter explores the current epizoology and etiology understanding of coral diseases reported across a broad geographical footprint and with repeated observations. As management of coral reefs pivots to looking for novel approaches to maintain ecosystem health, mitigate the impacts of rapid climate change, and build reef resilience, a better understanding of the environmental drivers and biological causes of coral disease is central to ensuring these iconic ecosystems persist into the future.