Bone remodeling involves the resorption of bone by osteoclasts and the synthesis of bone matrix by osteoblasts. Recently, an essential cytokine system for osteoclast biology has been identified and extensively characterized. This system consists of a ligand, receptor activator of NF-kappaB ligand (RANKL), a receptor, RANK, and its soluble decoy receptor, osteoprotegerin (OPG). RANKL, a member of the tumor necrosis factor (TNF) family, triggers osteoclastogenesis by forming a complex with RANK, a member of the TNF receptor family. Because members of the TNF family have the same topology and the extracellular domains of the TNF receptor family members also adopt the same structural scaffold, in addition to their rapid increase in the number, this poses an intriguing question of how recognition between cognate ligands and receptors is achieved in a highly specific manner. Structural studies on the mouse RANKL extracellular domain showed that the RANKL is trimeric, and each subunit has a beta-strand jellyroll topology like the other members of the TNF family. A comparison of RANKL with TNF-beta and TNF-related apoptosis-inducing ligand (TRAIL), whose structures were determined to be in the complex form with their respective receptor, revealed conserved and specific features of RANKL in the TNF superfamily. Residues important for receptor binding and activation have also been confirmed by mutagenesis experiments. Further structural and mutational studies on the RANKL/RANK/OPG system will provide useful information for developing drug candidates that inhibit osteoclastogenesis and mediate problems of bone metabolism.