Bone is a dynamic tissue that is constantly renewed. The cell populations that participate in this process--the osteoblasts and osteoclasts--are derived from different progenitor pools that are under distinct molecular control mechanisms. Together, these cells form temporary anatomical structures, called basic multicellular units, that execute bone remodeling. A number of stimuli affect bone turnover, including hormones, cytokines, and mechanical stimuli. All of these factors affect the amount and quality of the tissue produced. Mechanical loading is a particularly potent stimulus for bone cells, which improves bone strength and inhibits bone loss with age. Like other materials, bone accumulates damage from loading, but, unlike engineering materials, bone is capable of self-repair. The molecular mechanisms by which bone adapts to loading and repairs damage are starting to become clear. Many of these processes have implications for bone health, disease, and the feasibility of living in weightless environments (e.g., spaceflight).