For several decades deJong  first identified the mineral found in skeletal tissues as similar in structure to hydroxyapatite, Ca5(PO4)3OH(OHAp), this phase was considered to be the only significant crystalline calcium phosphate constituent in these tissues. It was not until 1957 that octacalcium phosphate, Ca8(HPO4)2(PO4)4 × 5H2O (OCP) was given serious consideration as an important hard tissue mineral component. In that year, Brown et al.  hypothesized that salt could be essential in the mineral-forming process as an intermediary phase with a unique intracrystalline relationship with the final OHAp-like phase. In this chapter, we will review Brown's proposal, which he more fully developed in his landmark 1962 papers [Brown, 1962; Brown et al., 1962] and the evidence used to support it, examine the currently available evidence for OCP in normal hard tissues and in pathological mineral deposits, and discuss the various possible mechanisms by which OCP could be involved in biological calcification processes to gether with supporting in vitro and synthetic studies.