Osteocalcin is a hydroxyapatite-binding protein, which is present in bone as the most abundant non-collagenous protein. Osteocalcin is exclusively synthesised by osteoblasts. In recent years, osteocalcin, which is secreted in the general circulation from osteoblastic cells, has been shown to have several hormonal features, and it has been shown to act either as a cytokine or as a chemoattractant for osteoblasts, osteoclasts and blood monocytes and to involve in endocrine regulation of glucose homeostasis. Thus, the novel role of osteocalcin in the regulation of many functions in the living body was found in recent studies. This book will be of help in the understanding of the wide aspects of the functional role of osteocalcin and its significance in disease.