The liver has a wide range of functions that may be disturbed in different ways by the many diseases which affect it and, in consequence, there are a large number of tests which look at different aspects of its function. Specific diagnoses are made using a range of clinical, biochemical, histological and radiological methods. Measurement of the plasma concentration of alanine aminotransferase (ALT, SGPT), gamma-glutamyl transpeptidase (gammaGT) and albumin are particularly valuable as these substances are specifically affected by liver disease. Their elevation can reveal increases in the membrane permeability of hepatocytes (ALT), cholestasis and toxic damage (gammaGT), or an impairment of liver protein synthesis and secretion (albumin), respectively. If their activities remain within the normal range the likelihood of significant liver disease is less than 2%. A series of quantitative liver function tests are described which each examine one aspect of hepatic function. Table 1 lists several situations where the assessment of hepatic function is necessary. Liver function tests are classified in Table 2.