The abdomen has been conveniently divided anatomically and clinically into specific intraperitoneal and extraperitoneal sites. A recognition of the pathways of spread and localizing features of infections, malignancies, and traumatic collections has immeasurably extended diagnostic imaging capabilities. Yet, while the abdomen may be visualized as subdivided into compartments, it must also be essentially appreciated that these compartments are an actual or potential interconnecting network. In the upper abdomen peritoneal reflections comprise 11 major ligaments and mesenteries. These are generally readily recognizable on computed tomographic scans by either their typical location and organ relationships or the landmarks provided by their major constituent vessels. These ligaments and mesenteries provide continuity of anatomic planes, which permits the spread of disease between intraperitoneal compartments, between extraperitoneal spaces, and between intraperitoneal and extraperitoneal localized sites.