Drugs used to treat osteoporosis act either by inhibiting bone resorption or stimulating bone formation. Osteoclast formation and bone resorption require cell-to- cell contact between osteoblasts and osteoclast precursors and osteoclasts in the marrow. Interaction between the receptor for activation of nuclear factor kappa B (RANK) on the surface of preosteoclasts and osteoclasts and RANK ligand on the surface of osteoblasts is required to stimulate osteoclast formation and activation. Binding of the RANK ligand to its receptor and osteoclastogenesis are prevented by osteoprotegerin (OPG), a decoy receptor produced by osteoblasts and marrow stromal cells. Thus, interference in binding of the RANK ligand to RANK by OPG determines the rate of bone resorption. Antiresorptive drugs such as estrogen, raloxifene, bisphosphonates, salmon calcitonin, and osteoprotegerin increase bone mass by inhibiting osteoclast function and bone resorption. Osteoprotegerin is more potent since it also inhibits osteoclast formation. Raloxifene, a selective estrogen receptor modulator (SERM), is a member of a class of compounds that act through estrogen receptors and are agonists for bone, antagonists for breast and uterine tissue and may be cardioprotective. The drug was shown to prevent vertebral fractures. Alendronate and bisphosphonates are the only antiresorptive drugs that have been shown to decrease fracture rates for the hip in addition to spine and other sites. Bone morphogenetic proteins stimulate bone formation at local sites and are being developed to stimulate fracture healing. Parathyroid hormone (1-34) stimulates osteoblastic bone formation, markedly increases bone mass, prevents vertebral fractures and is under development to treat osteoporosis.