Bone is a metabolically active tissue that throughout life undergoes constant remodeling. Skeletal turnover is achieved by two counteracting processes: bone formation and bone resorption. Whereas bone formation is a function of osteoblast activity, bone resorption is attributed to osteoclast activity. The metabolic and cellular events in the skeleton are regulated by a large number of systemic and local modulators, such as parathyroid hormone, vitamin D, sex hormones, glucocorticoids, calcitonin, prostaglandins, growth factors, and cytokines (see Chap. 49, Chap. 50, Chap. 51, Chap. 54, Chap. 55 and Chap. 173).