Calcareous nannofossils have been established as a fundamental means by which biostratigraphic correlation of Cenozoic marine sediment can be made. They are commonly present in cool-water areas and are especially diverse and consistently present in warm-water areas. The characteristic rapid evolution, abundance, and solution resistance of certain distinctive genera such as Discoaster and Sphenolithus permit the recognition of up to 56 Cenozoic zonal units in tropical areas. Diminishing diversity and generally shallower calcite-compensation depths reduce the resolution of the zonation in nontropical areas, most notably during the Neogene. Ocean-circulation patterns and plate-tectonic motions affect the geographic limits of nannofossil zonation.