Biological production of calcium carbonate in the oceans is an important process. Although carbonate is produced in the open ocean (pelagic, see Chapter 5), this chapter concentrates on production in coastal waters (neritic) because this contributes sediment to the coast through skeletal breakdown producing sand and gravel deposits on beaches, across continental shelves, and within reefs. Marine organisms with hard body parts precipitate calcium carbonate as the minerals calcite or aragonite. Corals, molluscs, foraminifera, bryozoans, red algae (for example the algal rims that characterize reef crests on Indo-Pacific reefs) are particularly productive, as well as some species of green algae (especially Halimeda). Upon death, these calcareous organisms break down by physical, chemical, and biological erosion processes through a series of discrete sediment sizes (Perry et al., 2011). Neritic carbonate production has been estimated to be approximately 2.5 Gt year-1 (Milliman and Droxler, 1995; Heap et al., 2009). The greatest contributors are coral reefs that form complex structures covering a total area of more than 250,000 km2 (Spalding and Grenfell, 1997; Vecsei, 2004), but other organisms, such as oysters, may also form smaller reef structures.