At the time of admission, 4 to 8% of all hospital admissions have a elevation of alkaline phosphatase. This high incidence of alkaline phosphatasemia has resulted in not only a need to identify the source of the elevated alkaline phosphatase; it has also led to a marked proliferation in laboratory procedures aimed at identifying the various isoenzymes of alkaline phosphatase. These various techniques are reviewed and their relative merits are discussed. No single technique adequately delineates all of the alkaline phosphatase isoenzymes, but the combination of heat fractionation, electrophoresis, and the measurement of an additional hepatobiliary enzyme has been shown to be able to identify the source of origin of more than 95% of all cases of alkaline phosphatasemia. The individual isoenzymes of alkaline phosphatase are discussed in detail, and the underlying pathophysiology is reviewed so that physiologic elevations of alkaline phosphatase are not confused with pathologic increases in alkaline phosphatase.