The tumor-node-metastasis (TNM) classification describes the anatomic extent of cancer. The ability to separately classify the individual T, N, and M elements and then group them into stages differs from other cancer staging classifications, which are primarily concerned with summarized groups. The objectives of the TNM system are to aid clinicians and investigators in planning treatment, assessing prognosis, stratifying patients for therapeutic studies, evaluating the results of treatment, and facilitating communication. The most important challenge facing TNM is how to interface the current taxonomy with the numerous nonanatomic prognostic factors currently in use or under study. As nonanatomic prognostic factors become widely used, TNM will remain a solid foundation on which to build prognostic classifications. There is, however, a risk that this system will be corrupted by a variety of irrelevant prognostic data. An anatomic extent of disease classification is needed to provide a standard against which to measure the importance of nonanatomic factors. Methods are needed to express overall prognosis without losing the vital anatomic content of TNM. These methods should be able to integrate multiple prognostic factors, including TNM, yet permit TNM to remain intact and distinct.