An ideal battery of tests would include indices of bone resorption and formation. They should be unique to bone, reflect total skeletal activity, and should correlate with traditional measures of bone remodeling activity, such as radiocalcium kinetics, histomorphometry, or changes in bone mass. Factors that confound their measurement, such as circadian rhythms, diet, age, sex, bone mass, liver function, and kidney clearance rates, should be clearly defined (Fig. 9). To date, no bone marker has been established to meet all these criteria, and each marker may have its own specific advantages and limitations. There are still questions that must be answered before there can be complete confidence in the information gained from measurement of any of the bone markers. Furthermore, it should be emphasized that none of the markers are diagnostic for any particular bone disease and cannot be used for this purpose in individual patients. Nevertheless, recent advances in research and development have provided assays with increased specificity, sensitivity, and availability. Because of this, bone markers can be used for a variety of important purposes: as tools for basic bone biology research, for defining general physiological phenomenon in clinical studies or drug trials, and for following individual patients.