Bone mass is the most important determinant of fracture risk. Current bone mass of an individual will be determined by the peak bone mass achieved in early adult life and the subsequent duration and rate of bone loss. In attempting to predict an individual's future risk of fracture it is therefore logical to attempt to assess both of these parameters. Serial measurements of bone mineral density and estimation of the rate of bone turnover may also be used to determine the response to treatment. In this chapter we review the currently available methods of measuring BMD and bone turnover, and discuss their place in the diagnosis and management of osteoporosis.