The cell that breaks down bone—it dissolves old or damaged bone so osteoblasts can replace it with new tissue.Osteoclasts are large, multinucleated cells derived from monocyte/macrophage lineage that resorb bone through acidification and proteolytic enzyme secretion (cathepsin K, TRAP, MMPs). Osteoclast activity is regulated by RANK/RANKL/OPG signaling. Excessive osteoclast activity relative to osteoblast formation drives osteoporosis and Paget’s disease. Calcitonin directly inhibits osteoclast function via calcitonin receptors on the osteoclast surface—the basis for its therapeutic use in bone diseases.