[Use and problems of the TNM classification]

The TNM system is the most important classification for describing the anatomical extent of tumors. The residual tumor classification describes the completeness of tumor removal. Although the definitions of both systems seem clear and precise, the daily work offers questions on how to use special situations. Despite the importance of the TNM- and residual tumor classification in planning clinical management before and after treatment, the misinterpretations and inconsistencies in application diminish or abrogate their clinical utility. Some of the questions and problems will be addressed in this contribution.