Osteoporosis is often described as a dynamic imbalance in the activity of osteoblasts, bone forming cells, and osteoclasts, cells that dissolve bone tissue. This imbalance creates regions of low bone mass with an architecture that is poorly suited to its mechanical function and leads to a dramatically increased fracture risk. The development of effective therapies requires an understanding of local effects including cellular-level phenomena and the effects of mechanical loading and damage as well as system effects such as parathyroid hormone and estrogen levels in the bloodstream. Perhaps even more importantly, these processes occur over many different time scales and the full effects of osteoporosis may not become apparent for decades.