Osteoclastogenesis – formation of polynuclear cells capable to resorb bone matrix, is tightly regulated by the number of signaling pathways. Fusion of bone marrow-derived macrophages, controlled by cytokines and osteoblast-bound ligands, leads eventually to formation of osteoclast. The structure of this cell and its function are discussed. Effects of drugs, cytokines and certain clinical conditions on formation of osteoclasts are reviewed in this paper.