Resorption of bone occurs continuously throughout life, first as part of skeletal growth and modeling and, later, in the process of bone-remodeling in the adult skeleton. Since the 1970's, considerable progress has been made in unraveling the basic cellular mechanisms that regulate the formation and activity of the osteoclast, the main cellular agent of bone resorption. These recent advances have resulted mainly from the introduction of new methods of isolation and culture of osteoclasts and evaluation of resorptive activity; techniques have also been developed for the generation of osteoclasts in long-term culture of precursor cells found in hematopoietic tissues and peripheral blood. Although reports have suggested that other cells (such as tumor cells or macrophages) are capable of degrading bone matrix and releasing local factors that contribute to resorption of bone, the osteoclast is the only cell specialized for this function.