The hepatorenal syndrome is defined as functional renal failure in advanced chronic or acute liver disease with portal hypertension. Morphologic abnormalities of the kidneys are frequently absent and tubular function is preserved. Patients with the hepatorenal syndrome are characterized by progressive splanchnic and systemic vasodilation and decreased effective arterial blood volume. Compensatory activation of vasoconstrictory systems maintains systemic hemodynamic stability but causes progressive afferent renal vasoconstriction, leading to reduction of glomerular filtration rate. Renal failure may be rapidly progressive (type I hepatorenal syndrome, frequently associated with spontaneous bacterial peritonitis) or may develop more slowly (type II). Orthotopic liver transplantation is the best current treatment and leads to a gradual recovery of renal function in the vast majority of patients. Because mortality of type I hepatorenal syndrome is excessive, supportive treatment by vasoconstrictor drugs, transjugular intrahepatic portosystemic shunt, and renal replacement therapy has been investigated to achieve stability until transplantation. The definite role of these promising developments, however, is still uncertain, emphasizing the need for large prospective multicentric investigations.