[The TNM system: only 3 letters for a rich but sometimes ambiguous language].

The T (tumor), N (node) and M (metastasis) staging system classifies tumors according to the anatomic extent. Since its first publication in 1940, the TNM classification has been periodically updated. This article reviews the main changes of the last edition and potential practical difficulties in interpretation of pN0 status, residual tumor, definition and classification of micrometastases and isolated tumoral cells, yTNM, use of X classification.