There are three classifications that describe the quantitative prognostic indicators of peritoneal dissemination for gastric cancer. The Japanese classification (P1, P2, and P3, Lyon classification, (stage I, II, stage III, and stage IV), and the Peritoneal Cancer Index (PCI). Carcinomatosis with limited extent (P1/ P2) corresponds to the PCI less than 13 and the stage I and II from Lyon classification. Carcinomatosis with large extent (P3) corresponds to PCI of 13 or larger and stage III and IV from Lyon classification. PCI enables one to describe the precise distribution of peritoneal dissemination. All three classifications correlate with prognosis. With regard to the surgical cytoreduction of the primary tumor and the peritoneal dissemination, Sugarbaker proposed the classification of completeness of cytoreduction (CCR). Patients with no macroscopic residual tumor had significantly better prognosis than those with residual disease. CCR is a valuable prognostic indicator after cytoreductive surgery.