Cathepsin K is essential for normal bone resorption; humans lacking cathepsin K exhibit pycnodysostosis, which is characterized by short stature and osteosclerosis. Cathepsin K knockout mice develop osteopetrosis and display features characteristic of pycnodysostosis, and osteoclasts isolated from these mice exhibit impaired bone resorption in vitro. Bone resorption depends upon the synthesis of cathepsin K by osteoclasts and its secretion into the extracellular compartment at the attachment site between osteoclasts and the bone surface, wherein the organic matrix is subsequently degraded by cathepsin K. Factors that directly modulate osteoclastic bone resorption, including cytokines (RANK ligand, tumor necrosis factor-alpha and interferon gamma), hormones (retinoic acid and estrogen) and nuclear transcriptional factors (c-jun and Mitf) also regulate cathepsin K gene expression. Osteoporosis is one of the leading causes of morbidity in the elderly and is characterized by a persistent excess of osteoclastic bone resorption. Therefore, cathepsin K is an attractive target for therapeutic intervention to prevent and ameliorate the significant deleterious impact of osteoporosis.