Coral reefs support the most diverse ecosystems in the ocean, largely because of the complex structures they build. However, climate change and ocean acidification threaten their very existence. This session will explore the chemical, biological, and physical factors that control reef growth, and how climate change and ocean acidification are likely to affect these processes. Coral reefs form over thousands of years. The rate of growth depends on how much calcium carbonate is produced by reef-building organisms (mainly corals and calcareous algae) balanced against how much reef material is removed through erosion and chemical dissolution. Reefs build when the rate of carbonate production outpaces the rate of carbonate removal, and shrink when the reverse happens. Some estimates suggest that most reef building will reverse by the middle of this century. Global warming has already reduced the abundance of corals and other reef-builders. For those organisms that remain, ocean acidification is slowing their rate of carbonate production while hastening both bioerosion and carbonate dissolution. However, even under natural conditions, reef growth varies dramatically from region to region, and vast differences are expected in how reef building responds to climate change in the future. Indeed, some reefs have probably already shifted from a state of construction to one of destruction. If so, what will this mean for reef ecosystems and for human communities that depend on them?