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 matrix48104152 and releasing local factors that contribute to resorption of bone45102105113, the osteoclast is the only cell specialized for this function. The purpose of this review is to integrate new knowledge regarding the formation of osteoclasts and the regulation of their activity within a general view of the cellular biology of normal and pathological bone resorption. After a review of the criteria by which osteoclastic cells are defined, the origin and formation of osteoclasts will be considered, along with mechanisms governing their recruitment, activation, and function. The manner in which these mechanisms contribute to osteoclastic bone resorption under various pathological conditions as well as the possible roles of other cells (both skeletal and extraskeletal in origin) in this process will be discussed.