Receptor activator of nuclear factor-κB (RANK), RANK ligand (RANKL) and its decoy receptor osteoprotegerin (OPG) play key roles in regulating bone turnover. They are involved in the mechanism of "crosstalk" between osteoblasts and osteoclasts. After binding with RANK, RANKL induces bone loss, whereas OPG prevents RANKL-RANK interaction and increases bone mass and strength. The expression of RANKL has been found on the surface of many cell types including cells of the osteoblast lineage, osteocytes, activated T cells and vascular endothelial cells. RANK is a cell bound receptor for RANKL. Its expression has been detected mainly on cells of the macrophage/monocytic lineage, including pre-osteoclastic cells. OPG is produced and secreted by many different cell types including osteoblasts and vascular cells. The OPG/RANKL/RANK system is regulated by many hormones and cytokines among which parathormone (PTH) is one of the most important.