Up-to-date clinical oncology requires an exact and standardized way of classifying tumors. Experience shows that mistakes can occur more frequently than generally expected during the classification procedure and documentation. Several possible sources of errors will be mentioned in this contribution. Errors can occur in localizing the exact site of the primary tumor, which can lead to an incorrect classification of metastases in lymph nodes, e.g., N1/pN1 instead of M1/pM1. Difficulties in histological typing of liver tumors, particularly in the differential diagnosis between hepatocellular carcinoma (HCC) and cholangiocarcinoma (CC) can be minimized by using immunohistochemistry. The grading of HCC can be performed according to two systems, namely those of Edmondson and Steiner and of the WHO. The grading system used should be indicated. For estimating the anatomical extent of primary liver tumors, vascular invasion is an important parameter. Resection margins have to be thoroughly examined to aim at an most exact residual tumor classification.