Vascular calcification is the major cause of cardiovascular morbidity and mortality in patients with type 2 diabetes mellitus, chronic kidney disease, and in aging patients.1 Considerable progress has been made in the past 2 decades in understanding the molecular mechanisms of vascular calcification.2–4 Regardless of the morphology and location, most evidence indicates that vascular calcification involves an organized process recapitulating many cellular and molecular events that govern skeletal bone formation. Although a large body of evidence shows that osteoblastic and osteochonrocytic cells contribute to vascular calcification, it remains unclear how osteoclasts are differentiated from their precursors and how osteoclasts play a role in calcium reabsorption in calcifying arteries. Osteoclasts develop from monocytic precursors of the hematopoietic lineage, and 2 distinct signaling systems are both necessary and sufficient for the early differentiation into multinucleated osteoclasts as demonstrated by osteopetrotic mice models with loss-of-function mutation of either gene.5–7 One is a signaling system that involves macrophage colony-stimulating factor (MCSF) and its receptor c-fms, and the other involves receptor activator of nuclear factor κB (RANK) ligand (RANKL), RANK, and osteoprotegerin (OPG), a soluble decoy receptor for RANKL.8–10 MCSF and RANKL induce osteoclast differentiation, fusion, and maturation on binding to its specific receptors, c-fms and RANK, respectively, on the surface of preosteoclastic monocytes. Increased production of RANKL in atherosclerotic lesions, particularly in endothelial cells and vascular smooth muscle cells (SMCs), has been well documented.11–13 See accompanying article on page 626 Studies of …