Histological type, malignancy grade, and tumor stage are among the most important parameters predicting outcome in cancer patients. Making use of immunocytochemistry as well as polymerase chain reaction-based techniques the demonstration of micrometastatic tumor spread, for example, into bone marrow, lymph nodes, and peritoneal cavity, is a new staging parameter of prognostic significance. In contrast, the prognostic value of different proliferation markers such as Ki67 (Mib 1), PCNA, and AgNOR has not yet been unequivocally established. A series of genetic change has been described in the development of cancer. In general, these changes seem to be of predictive value within defined tumor stages and it might be helpful to determine several genetic lesions within one tumor. Very recently a new mechanism of carcinogenesis closely related to the hereditary nonpolyposis cancer syndrome (HNPCC) was detected. Due to mutations in mismatch repair genes (hMSH 2, hMLH1, hPMS1,2) instabilities in simple repetitive genomic sequences occur, which are the genetic hallmark of most HNPCC tumors. This opens a new field to cancer prevention.