At present, the pathologist is an important member of the treatment team who provides information and consultation for selecting treatment, estimating prognosis and evaluating outcome. The pathologist's functions include pretreatment microscopic diagnosis, intraoperative consultation and histopathologic examination of tumor resection specimens. Tumor classification follows international rules published by the UICC and the WHO. The strongest predictors of outcome are the residual tumor (R) classification and the TNM/pTNM classification of anatomic extent before treatment. The new 5th edition of TNM (1997) contains some changes in the classification of gastric, colorectal and exocrine pancreatic carcinoma. There are some pathological features which independently influence prognosis in addition to R and TNM. For the most biological and molecular markers the prognostic significance remains to be proven. In future, the main objective of prognostic factor research will be the development of prognostic systems.