As the inorganic constituents of skeletons, dentine and the enamel of teeth in all vertebrates, as well as antlers of male deer, calcium orthophosphates (CaPO4) appear to be the key materials to sustain all life on Earth. Therefore, biologically relevant CaPO4 possess all the necessary features of the biomaterials, such as biocompatibility, bioactivity, bioresorbability, osteoconductivity, osteoinductivity, and appear to be non-toxic, non-inflammatory and non-immunogenic. In this book, the author presents current state-of-the-art applications on the occurrence, major properties and biomimetic crystallisation of CaPO4, as well as information on their history. Topics discussed include the geological and biological occurrences, a brief description of all known members of the CaPO4 family, their presence and major functions in the hard tissues of living organisms as both desired (normal) and undesired (pathological) calcifications, as well as the available information on biomimetic crystallisation. The detailed description of the historical development of our knowledge on CaPO4 is given in the second section of this book.