Osteoclasts are derived following the fusion of precursors of hematopoietic and myelomonocytic origin after appropriate stimulus, such as that afforded by RANKL and M-CSF. Thus the osteoclast can be considered as a specialized type of macrophage, and several of the factors that affect osteoclast formation also have affects upon macrophage differentiation. Inhibitors of osteoclast formation may perturb RANKL or M-CSF signalling or affect other signalling pathways. Several of these inhibitors are discussed with the view of their capacity to influence osteoclast differentiation, but not necessarily their activity. (C) 2004 Elsevier Inc. All rights reserved.