Mineralization processes in the body are controlled by physicochemical and cellular regulation of hydroxyapatite (HA) nucleators and inhibitors. The chemical mechanism of action of HA inhibitors has been studied in vitro using solution pH-stat techniques or Types I and II collagen gel diffusion systems. Three biologically relevant systems are used with these methodologies: (1) transformation of amorphous calcium phosphate (ACP) to crystalline HA; (2) direct formation of HA; and (3) growth of HA crystals. Several different mechanisms have been identified for HA inhibition. Condensed phosphates (containing P-O-P linkages) and diphosphonates (containing P-C-P linkages) bind strongly to the surface of forming HA nuclei and crystals and poison growth sites at concentrations as low as 10(-6) M, blocking HA formation. From this in vitro work, diphosphonates have been developed for the treatment of Paget's disease. Proteoglycans, found in cartilage, delay HA formation by a steric effect whereby large volumes of solution become unavailable for HA formation and growth as these enormous macromolecules tumble about. Mg ions enter the structure of forming HA nuclei by replacing Ca, resulting in a distorted atomic structure that slows subsequent growth to HA. Al ions delay HA formation, not by entering the structure of forming HA nuclei, but by adsorbing on the surface of growing HA crystals. Serum proteins slow the transformation of ACP to HA by adsorbing on the ACP surface, which decreases its dissolution rate. Metal-citrate complexes can inhibit HA formation and growth at concentrations as low as 10(-5) to 10(-6) M. Phosphorylated molecules such as acidic proline-rich phosphoproteins and statherins found in saliva suppress HA crystal growth on tooth surfaces by adsorbing on active surface sites. Future research in this field lies in the study of interactions of HA inhibitors found together in calcifying tissues.