An increasing amount of data indicate that osteocalcin is an endocrine hormone which regulates energy metabolism, male fertility and brain development. However, the detailed functions and mechanism of osteocalcin are not well understood and conflicting results have been obtained from researchers worldwide. In the present review, we summarize the progress of osteocalcin studies over the past 40 years, focusing on the structure of carboxylated and undercarboxylated osteocalcin, new functions and putative receptors, the role of osteocalcin in bone remodeling, specific expression and regulation in osteoblasts, and new indices for clinical studies. The complexity of osteocalcin in completely, uncompletely and non-carboxylated forms may account for the discrepancies in its tertiary structure and clinical results. Moreover, the extensive expression of osteocalcin and its putative receptor GPRC6A imply that there are new physiological functions and mechanisms of action of osteocalcin to be explored. New discoveries related to osteocalcin function will assist its potential clinical application and physiological theory, but comprehensive investigations are required.