Abstract Modern marine carbonate sediments may be roughly grouped into three main classes: deep-water oozes, organic (“coral”) reefs and associated deposits, and continental shelf lime- sands and lime-muds; the last two overlap somewhat. Carbonate ooze covers vast areas on the bottoms of the three great oceans, especially in low latitudes where the water is less than 3 miles deep. Tests of pelagic foraminifera are the principal component of at least the purer oozes, though pteropod shells and coccoliths are locally important. Clayey carbonate ooze occurs in somewhat deeper water and also toward coasts where there is much terrigenous sediment; it is common in the deeper basins of the world’s mediterraneans. Such ooze probably did not form before the Cretaceous; the accessible geologic record apparently contains nothing like the ooze of the open ocean but may contain deposits like those in the modern mediterraneans. Organic reefs are practically confined to low latitudes; atolls dot the open oceans but the largest reef areas are on continental margins, as in the East and West Indies. Here the reef deposits are associated with wide areas of shelf carbonate deposits, partly chemical but probably in larger part fragmental-organic. Deposits of this type, with local areas of reefs, have been common since the Middle Ordovician on shelves and in epeiric seas; before that, chemical precipitation probably played a relatively more important part. Calcium carbonate has been deposited in great volume since far back in the Precambrian, but the evolution of organisms lias twice changed the dominant kind of deposit. With the evolution of benthonic shellfish in the early Paleozoic, chemical deposits gave way in large part to shallow-water organic deposits. With the evolution of pelagic foraminifera in the Cretaceous, deep-sea oozes became important, interfering with the previous balance between ocean and continent by withdrawing CaC03 from circulation. Apparently, shallow-water carbonate deposits were more common during the late Precambrian, Paleozoic, and Mesozoic than during the Cenozoic or today.