Corals are animals belonging to the Phylum Cnidaria (formerly Coelenterata) which also includes jellyfish, sea anemones and sea pens. Reefs are built by corals of the Order Scleractinia. These secrete skeletons of calcium carbonate and, as the corals grow in large colonies, these accumulate over time to form reefs.

Reef-building corals normally occur in depths of less than 50m where sunlight penetrates. This is because many corals derive their nutrition from zooxanthellae, microscopic photosynthetic algae living symbiotically in the tissues of the polyps. The reef-builders require water temperatures of 20-28°C and so are located almost entirely between 30°N and 30°S. Reefs include atolls which are rings of coral islands around a central lagoon, fringing reefs on the borders of tropical shores and barrier reefs separated from a land mass by a significant channel. Changes in sea level or subsidence of the land have contributed to the characteristics of atolls and barrier reefs. The atolls appeared where reefs have been created around an island which has subsequently become submerged, principally by the rise in sea levels since the last ice age, while the latter have formed as the original coastlines of larger land masses have retreated. The persistence of these reefs in the event of future sea level rises depends on the rate of growth of the reef keeping pace with the change in sea level. The corals cannot survive in high turbidity or low salinity.

Corals grow best and reefs thrive on areas exposed to strong wave action which keeps the reef clear of sediment, distributes food, nutrients, and oxygen to the reef and disperses coral larvae. The reefs protect the adjacent shores from the impact of wave action, benefits the local communities by providing rich fisheries and attracting tourists as well as being a source of food and medicines. Coral reefs are considered to be worth one billion dollars each year to the economy of south Florida alone.

Reefs are under threat from direct pressures such as overfishing (including the use of explosives), sediment deposition due to dredging, deforestation and other changes in land use, increasing turbidity through nutrient input and pollution by toxic chemicals. These threats are compounded by the effects of increasing carbon dioxide concentration and climate change. The higher CO₂ levels are already causing a decline in the calcification rate of corals and in future could pass a threshold where reef building is no longer possible.

Rapid sea level rise in the event of large-scale melting of the ice-caps may outstrip the rate at which coral can grow, causing the death of reefs as light penetration to the polyps declines below the levels needed by the zooxanthellae. Athough corals thrive in warm water, the record temperatures in recent years have caused 'coral bleaching', death of the algae which provide the corals with their nutrition and colour. Although recovery from short-term damage is possible, mass deaths of corals are increasingly occurring following bleaching. In 1998, the warmest year of the 20th century, 16% of the world’s reefs were seriously damaged and only 40% of these are reported to be recovering well. Overall 24% of the world’s reefs are immediately threatened with collapse through anthropogenic pressures, and a further 26% are under longer term threat.

Precipitation of calcium carbonate (CaCO₃) by corals and calcerous plankton removes inorganic carbon from the upper layer of the ocean as carbonate ion (CO₃^2-) and counter intuitively CO₂ is outgassed due to a consequent reduction in alkalinity. In summary, as more calcium carbonate is produced more CO₂ is added to the atmosphere. In the future however, the expected increase in atmospheric CO₂ will markedly decrease the pH of seawater and reduce the precipitation of CaCO₃ (calcification). This will have detrimental consequences for corals and calcifying plankton and ecosystems that depend on them, but it will increase the amount of anthropogenic CO₂ that the oceans can store.