Despite intensive investigations the calcification process is still poorly understood. The great number of theories which have been and are still being proposed reflect the complexity of the subject and the uncertainty of our knowledge. In this review an attempt will be made to sum up knowledge of the nature of the biological mineral and the mechanism(s) of its formation. Emphasis will be placed on the inorganic part of the calcified tissues, although it must be kept in mind that mineral never deposits without an organic matrix which represents an appreciable proportion of the total weight.