Osteoclasts are large, multinucleated cells formed by the fusion of hematopoietic, mononuclear progenitors of the monocyte/macrophage lineage, and are the cells responsible for resorbing bone. Osteoclasts are usually few in number relative to other cell types in bone and are difficult to isolate because they are contained in a hard tissue; in addition, they are at the end of their proliferation and differentiation cycle, presenting problems for the creation of osteoclast cell lines. However, with the development, almost 20 years ago, of in vitro resorption pit formation models, using isolated primary, mature osteoclasts and mineralized bone or dentine matrix as a substrate (1,2), considerable progress was made in our understanding of osteoclast biology. Data from such short term cultures complements that obtained from bone organ culture resorption models and long-term cultures of osteoclast forming hematopoietic stem cells derived from marrow or peripheral blood.