Bone resorption is required for skeletal modelling during bone growth and for mineral homeostasis and bone remodelling throughout life. Osteoclasts are multinucleated cells that are uniquely specialised to carry out this physiological bone resorption. As osteolysis is a feature of most diseases of bone and joint, osteoclasts also play a role in pathological bone resorption, the extent of which is a function of the cellular and molecular mechanisms that govern their formation and function.