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Anticancer agents go through a process by which their antitumor activity, on the basis of the amount of tumor shrinkage they could generate, has been investigated. In the late 1970s, the International Union Against Cancer and the World Health Organization (WHO) introduced specific criteria for the codification of tumor response evaluation. In 1994, several organizations involved in clinical cancer research joined together to undertake the review of these response evaluation criteria on the basis of their experience and knowledge. After several years of intensive discussions, new guidelines are ready and will replace the previous WHO criteria. In parallel to this initiative, one of the participating groups developed a model by which response rates could be derived from unidimensional measurement of tumor lesions instead of the usual bidimensional approach. This new concept has been largely validated by the Response Evaluation Criteria in Solid Tumors (Recist) Group and integrated into the present guidelines. This special article provides some philosophic background to clarify the various purposes of response evaluation. It proposes a model by which a combined assessment of all existing lesions, characterized by target lesion (to be measured) and nontarget lesions, is used to extrapolate an overall response to treatment. Methods of assessing tumor lesions are better codified. All other aspects of response evaluation have been discussed, reviewed, and amended whenever suitable.