Physiological bone turnover can be divided into 2 temporal phases: modeling, which occurs during development (a topic not addressed in this series; for recent reviews see refs. 31, 32), and remodeling, a lifelong process involving tissue renewal. Remodeling starts with removal by osteoclasts of matrix, a mixture of insoluble proteins in which type I collagen is predominant (>90%) and a poorly crystalline, chemically modified hydroxyapatite. Following resorption, osteoblasts are recruited to the site, where they secrete and mineralize new matrix. Until about age 30–35 bone replacement exceeds or equals removal, thus increasing or maintaining bone mass; thereafter, bone mass decreases, reflecting the predominance of osteoclast activity. The major thrust of the bone-related Review articles contained within this series is to outline selected new and important aspects of osteoblast and osteoclast biology (for an excellent review of the pathophysiology of osteoporosis see ref. 33).