This article reviews the structure, methods for characterisation and deposition of hydroxyapatite and related calcium phosphates from biological fluids. The importance of the deposition of hydroxyapatite for the normal biological calcification of bones and teeth is discussed, as is the pathological calcification of tissues and the formation of calculi. However, the emphasis is on the deposition of hydroxyapatite on synthetic materials of devices which are inserted or implanted into the body during the course of medical treatment. Urinary catheters are used as the major example of these devices. The problem of catheter encrustation, the identification of hydroxyapatite in the encrusting deposists, the mechanism of encrustation (involving infection by bacteria which produce urease), the importance of nucleation and the dissolution of deposits are described.