The portal vein and hepatic arteries provide blood to the liver, where they course together. The main portal vein is formed by the junction of the splenic and superior mesenteric veins and branches into the right and left portal veins. Both resistance to blood flow in the portal vein and portal vein pressure increase in patients with portal hypertension, leading to development of collateral circulation. However, this compensatory mechanism usually fails to normalize portal pressure as resistance in the liver continues to increase. Portal hemodynamic changes, from normal hepatopetal flow (toward the liver) to hepatofugal flow (way from the liver), set up a back-and- motion within the portal vein as portal pressure increases. As in response to increase portal pressure, the existing veins enlarge to shunt blood flow from the portal vein to the systemic vein, either bypassing the liver parenchyma (intrahepatic shunts) or through extrahepatic collaterals. Extrahepatic portalsystematic collaterals may drain toward either the superior or the inferior vena cava. The former accounts for esophageal varices, gastric varices, or both; the latter, for splenorenal shunts, the paraumbilical vein, and other pathways through the peripancreatic or pericolic veins. This chapter further describes the most appropriate scanning methods for demonstrating these collaterals and other portal vein abnormalities, including portal vein thrombosis, portal aneurysm, portal vein-hepatic vein shunt, and portal vein gas. We emphasize interpretation of sonographic images.