The clinical response to treatment is an important indicator of the therapeutic effect of anticancer agents. Its value and interpretation has to be carefully considered within the context that it is used. In daily practice, response assessment is combined with other indicators of the patient's condition to contribute to the decision-making process. In clinical trials, it is widely used to identify and quantify the anti-tumour activity of new agents. In this context, response evaluation is conducted on the basis of strict predefined criteria such as the World Health Organization (WHO) or Response Evaluation Criteria In Solid Tumors (RECIST) criteria. The RECIST criteria have recently been proposed and offer a detailed guidance to perform a response evaluation. Clinical response is also used as an indicator of therapeutic efficacy in combination with other indicators. Its value as a surrogate indicator of a survival benefit remains unclear in most instances and can hardly be established within the framework of a single randomised trial. With the development of new anticancer agents that behave differently to cytotoxics, clinical benefit will have to integrate concepts of disease stabilisation or time to progression. Over the next decade, oncologists will be able to assess the biological response before the clinical response, and a lot of work and energy will have to be dedicated to assess the predictive and, possibly, the prognostic value of the biological response with regard to the clinical response, as well as more definitive measures of clinical benefit.