Calcium phosphates comprise the largest group of biominerals in vertebrate animals. They also have many uses in industry, medicine and everyday life. Orthophosphates are salts of the tribasic phosphoric acid which include H2PO4-, HPO42- and PO43- ionic species. The phosphates containing HPO42- and PO43- generally constitute the biologically relevant calcium phosphates. Salts with only H2PO4- are not normally found under physiological conditions, but are commercially important as components in fertilizers. The basic chemical and physical properties of a material depend on the kinds and relative amounts of the consituent atoms and their locations relative to one another. Details of the atomic-scale architecture can be used to provide a basic step in understanding the characteristics of these materials. This review of crystal structures of calcium phosphates will focus on their structure-related properties relevant properties relevant to biomineralization. The discussion will be based on the structural types, rather than on the ionic species involved or the associated Ca/P molar ratio.The known pure calcium phosphates can be classified into three major structural types: (1) the apatite type, Ca10(PO4)6X2, which includes the derivatives of hydroxyapatite (X=OH) and fluorapatite (X=F) as well as those related to apatite-type structures, such as octacalcium phosphate (OCP), Ca8(HPO4)2(PO4)4 5H2O and tetracalcium phosphate (TTCP), Ca4(PO4)2O;(2) the glaserite type, which can be considered to include all polymorphs of tricalcium phosphates (TCP), Ca3(PO4)2, and (3) the Ca-PO4sheet-containing compounds, which include dicalcium phosphate dihydrate (DCPD), CaHPO4 2H2O, dicalcium phosphate anhydrous (DCPA), CaHPO4, and monocalcium phosphates, Ca(H2PO4)2 H2PO4)2 struviet-type structures, after the biomineral struvite, Mg(NH4)PO4 6 H2O, have been included as a new type of calcium phosphate [Dickens and Brown, 1972], although this group does not represent a pure calcium phosphate, but they will not be discussed here. Amorphous calcium phosphate, which may be related to one or more of the structural types discussed above, has been considered as a special class of calcium phosphate in Eanes [2001, pp. 130-147, this volume].