Although the segmentation of the liver appears straightforward, successful segmentectomy or lobectomy depends upon a thorough understanding of the difference between the portal vein, biliary duct, and hepatic artery distribution as opposed to the hepatic vein drainage. In general, the portal triad structures bifurcate in a serial manner and ultimately lead directly into each of the eight areas. The specific exception to this rule is the paraumbilicalis of the left hepatic branch of the portal vein, as this structure straddles the division between the left inferior medial and lateral segments. Thus, it lies roughly under the round ligament (Figure 1, 7). The superior and inferior areas of the left lateral lobe have a portal venous supply from either end of the paraumbilicalis (Figure 1, 9 and 10); however, special note should be made of the paired medial supply to the superior and inferior areas of the medial segment (Figure 1, 8 and 12). It is equally important at this point to examine the biliary and arterial supply of this area (Figure 6). The main left hepatic duct and artery proceed with the expected bifurcations out through the superior and inferior divisions of the left lateral segment; however, the left medial segment duct and artery (Figure 6, 13) do not divide and send a large branch to the superior and inferior areas, but rather send long, paired structures out in each direction from the junction of the two areas (Figure 6, 12 and 13).