Planktic foraminifera are single-celled marine eukaryotes characterized by having calcareous shells. They are holoplankton with 40-50 identified species in the world ocean. Their biology, diversity, and shell chemistry are sensitive to changes in the oceanic environment, and therefore their carbonate shells are useful climatic tracers of temperature, water mass, and other chemical indicators of global change. Production of carbonate shells plays an important role in marine biogeochemical cycles involving carbon and is closely related to the Earth's climate systems. Geochemical studies combined with culture experiments using planktic foraminifera provide much useful information about future biological and climatic responses on Earth. Molecular biology based on DNA has become a major tool for all ecological, biological, and evolutional studies of foraminifera. Molecular phylogenic analyses have revealed that morphologic classification of the planktic foraminifera resulted in underestimation of their biodiversity.