Corals reproduce predominantly sexually, with 25% of hermatypic corals (stony corals) forming single sex (gonochoristic) colonies and the rest hermaphroditic. About 75% of all hermatypic corals release gametes - eggs and sperm - into the water to spread colonies over large distances in what is called broadcast spawning. The gametes fuse during fertilization to form a microscopic larva called a planula, typically pink and elliptical in shape; a moderately sized coral colony can form several thousands of these larva per year to overcome the huge hazards that prevent formation of a new colony.

Corals that do not broadcast spawn are called brooders, with most non-stony corals displaying this characteristic. These corals release sperm but harbour the eggs, allowing larger, negatively buoyant, planulae to form which are later released ready to settle. The larva grows into a coral polyp and eventually becomes a coral head by asexual budding and growth to create new polyps.

Synchronous spawning is very typical on a coral reef, even when there are multiple species present, all the corals on the reef release gametes during the same night. This synchrony is essential so that male and female gametes can meet and form planula. The cues that guide the release are complex, but over the short term involve lunar changes and time of sunset, although chemical signalling has not been ruled out. In some places the coral spawn can be dramatic, usually occurring at night, where the usually clear water becomes cloudy with gametes.

Within a head of coral the genetically identical polyps reproduce asexually to allow growth of the colony. This is achieved either through gemmation or budding or through division, both shown in the diagrams of Orbicella annularis on the right. Budding involves a new polyp growing from an adult, whereas division forms two polyps as large as the original.

Whole colonies can reproduce asexually through fragmentation where a piece broken off a coral head and moved by wave action can continue to grow in a new location.