Skeletal mass is maintained by a balance between cells which resorb bone (osteoclasts) and cells which form bone (osteoblasts). Bone development and growth is an on-going, life-long process. Bone is formed during embryonic life, grows rapidly through childhood, and peaks around 20 years of age (formation exceeds resorption). For humans the skeleton then enters a long period, approximately 40 years, when bone mass remains relatively stable. Skeletal turnover continues but the net effect of resorption and formation on bone mass is zero. For women this ends when they enter menopause and similar bone loss occurs for men, but later in life. These opposite functions are coupled, resorption precedes formation, and osteoblasts, or their precursors, stromal cells, regulate osteoclast formation and activity. Until recently, the molecular nature of this regulation, was poorly understood. However, recent observations have identified members of the TNF family of ligands and receptors as critical regulators of osteoclastogenesis. Osteoprotegerin (OPG) a decoy receptor was first identified. Its ligand, receptor activator of nuclear factor-kappaB ligand (RANKL), was quickly found, and shown to be expressed on stromal cells and osteoblasts. Its cognate receptor, RANK, was found to be expressed in high levels on osteoclast precursors. The interaction between RANKL and RANK was shown to be required for osteoclast formation. These observations have provided a molecular understanding of the coupling between osteoclastic bone resorption and osteoblastic bone formation. Moreover, they provide a framework on which to base a clear understanding of normal (e.g. postmenopausal osteoporosis and age associated bone loss) and pathologic skeletal changes (e.g. osteopetrosis, glucocorticoid-induced osteoporosis, periodontal disease, bone metastases, Paget's disease, hyperparathyroidism, and rheumatoid arthritis).