ACP is a unique calcium phosphate in that it lacks long-range crystalline order. Yet the constancy in the composition of ACP over a wide range of solution conditions suggests a well-defined local structural unit. Although this order within disorder is the most distinguishing feature of ACP, the solution instability of ACP and its ready transformation into crystalline phases such as OCP and apatite may be of greater biological relevance. In particular, the initiating role ACP plays in matrix vesicle mineralization raises the importance of this phase from a mere laboratory curiosity to that of a key intermediate in skeletal calcification.