Much has been learned about the cell biology and molecular biology of the osteoclast in the last 5 yr. The osteoclast appears to be derived from CFU-GM, the committed monocyte-granulocyte precursor cell. This cell then differentiates into more committed precursors for the osteoclast. The role of the marrow microenvironment appears to be critical for murine osteoclast formation, although in human systems it appears to be nonessential but acts to enhance osteoclast formation and resorption. The osteoclast has been shown to be a secretory cell capable of producing both stimulators and inhibitors of osteoclast formation and resorption. The identification of the role of protooncogenes, such as c-fos and pp60c-src, in osteoclast differentiation and bone resorption has provided important insights into the regulation of normal osteoclast activity. Studies such as these should help us to dissect the pathophysiology of abnormal osteoclastic activity, such as seen in hypercalcemia of malignancy, osteopetrosis, and Paget's disease of bone. Future research is needed to further delineate the signaling pathways involved in osteoclastic bone resorption in response to cytokines and hormones, as well as to identify the molecular events required for commitment of multipotent precursors to the osteoclast lineage. Development of osteoclast cell lines may be possible and would greatly enhance our understanding of the biology of the osteoclast. Utilization of current model systems to examine the effects of cytokines and hormones on osteoclast precursors in vitro and in vivo and the ability to obtain large numbers of highly purified osteoclasts for production of osteoclast cDNA libraries should lead to important new discoveries in osteoclast biology.