The first report on a mesenteric cyst was published by an Italian anatomist in 1507.1 Since then, the origin and classification of mesenteric and omental cysts have been a matter of debate. Moynihan2 attempted in 1897 a differentiation of abdominal cysts on the basis of fluid content. Serous cysts are characterized by a translucent, straw-colored fluid of low specific gravity. Their chemical composition is similar to plasma. In contrast, chylous cysts contain an opaque fluid of high specific gravity, with lipids and fat globules contributing to the fluid content. Subsequent attempts at a more appropriate classification of intra-abdominal cysts have been based on suspected etiology initially proposed by Beahrs in 1950.3 However, the etiology of many intra-abdominal cysts is questionable, rendering classifications of this type of limited clinical usefulness. A more appropriate classification, based on histologic findings, was proposed in 1987 by Ros et al.4 This differentiation is applicable to all operative cases and can provide the basis for a more uniform evaluation of the clinical and pathologic characteristics of these cysts (Table 63.1).