The peritoneum is a mesothelial lining of the abdominal cavity (parietal) and intraperitoneal viscera (visceral). The cavity contains a small amount of fluid, which circulates from cephalad to caudal to cephalad, influenced by negative pressure under the diaphragm during exhalation, gravity, and bowel peristalsis. Peritoneal reflections and mesenteries divide the cavity into various compartments (supramesocolic, inframesocolic, and pelvis). These reflections support the peritoneal organs and provide vascular and nervous connections while within the cavity they influence pathway of intraperitoneal fluid circulation. Capillary force over convex surfaces influence stasis of fluid and promotes peritoneal seeding; thus, there are numerous areas in which peritoneal masses are seen more commonly. These areas include the undersurface of the diaphragm (negative pressure and capillary force), the omentum (bathed in fluid), the right lower quadrant (oblique course from left superior to right inferior of the small bowel mesentery), the left lower quadrant (transverse course of the sigmoid), and the pelvis (gravity). Peritoneal carcinomatosis may be either primary (mesothelioma) or metastatic. The mode of spread is by direct invasion, lymphatic permeation, peritoneal seeding or hematogenous. The imaging patterns include fibronodular stranding, nodules, plaques, and masses. Mesenteric thickening may produce pleated or stellate patterns. Spiral CT is the most useful modality in diagnosis and follow-up of peritoneal tumors.