Publisher Summary This chapter discusses various tests of function of liver. Apart from the hepatic parenchymal cells that carry out most of the liver functions, other types of cell line the biliary tract. The liver also contains cells of the reticulo-endothelial system. Different disease processes involve these cells in different proportions. Hepatic parenchymal cells are predominantly affected in hepatitis of any etiology—whether acute or chronic—and by infiltration of the liver, as with secondary malignant deposits. They are involved, together with reticulo-endothelial cells, in cirrhosis. They may be secondarily involved in obstructive biliary conditions because of regurgitation of bile; in this case, evidence of biliary involvement is predominant. It is suggested that the tests depend on detection of intracellular constituents in abnormal amounts in the plasma. Enzymes can be detected in very low concentrations because their catalytic activity can be measured.