Bone continually undergoes a process of renewal called remodeling. In the normal adult skeleton, bone formation and bone resorption are closely coupled: new bone laid down by osteoblasts exactly matches osteoclastic bone resorption. The bone mass of an individual in later life is a consequence of the peak bone mass accrued in utero and during childhood and puberty, as well as the subsequent rate of bone loss. Genetic factors strongly contribute in determining the peak bone mass. However, hormonal, nutritional, and environmental influences modulate the genetically determined pattern of skeletal growth. More than 1.5 million fractures related to osteoporosis occur in the United States every year. Ninety percent of all hip and spine fractures are related to osteoporosis. Most osteoporotic fractures involve the femoral neck, the vertebral bodies, or the wrist. Two types of scores have been used traditionally to quantify bone mineral density (BMD): The T score is the number of standard deviations of the patient's BMD above or below the young-normal mean BMD. The Z score is the number of standard deviations of the measurement above or below the age-matched mean BMD. The World Health Organization (WHO) defines osteoporosis with a T score ≤-2.5. © 2010 Springer-Verlag London.