Abstract: The coccolithophorids are unicellular marine algae belonging to the Haptophyta which have calcified scales, coccoliths, on their surface. These algae play an important role in maintaining the global carbon pool of the marine ecosystem, not only as a primary producer in the food chain, but also as a producer of CaCO3. Being different from other biomineralization systems, their biomineralization system has the unique characteristic of being intracellular. The mechanisms involved have been investigated mainly in the genera Emiliania and Pleurochrysis, which are included in the two separate phylogenetic lineages of the coccolithophorids. Acidic polysaccharides, which are present in the coccoliths, are considered to be involved in the coccolith formation. The structures and localization of acidic polysaccharides have been analysed in the two genera. However, the molecular mechanisms underlying the coccolith formation by coccolithophorids remain unclear. Recently, analyses of EST and genome sequences, and involving cDNA arrays have been started in E. huxleyi and P. haptonemofera. The morphological, biochemical, and molecular biological studies on the calcification of the coccolithophorids are reviewed, focusing on Pleurochrysis and Emiliania.