Geminal bisphosphonates are a new class of drugs recently developed. They are characterized by a phosphorus-carbon-phosphorus (PCP) bond and are thus analogs of pyrophosphate. The bind strongly to hydroxyapatite crystals and inhibit in vitro both their formation and dissolution. In vivo they inhibit soft tissue calcification and in some instances normal calcification. Bone resorption is also inhibited. The mechanism of action for the inhibition of calcification is probably the inhibition of calcium phosphate crystal growth. However, the mode of action on bone resorption is not yet known but is more likely to be cellular. These properties have been used in man to prevent heterotopic calcification, to retard dental calculus, and to slow down bone resorption in conditions such as Paget's disease, tumoral bone disease, and osteoporotic immobilization.