Response to treatment can be subjective or objective. A subjective response is one perceived by the patient in terms of, for example, relief of pain, dyspnoea or improvement in appetite, weight gain or energy. Quantitative measurements of these subjective symptoms form an increasingly important part of the assessment of response to chemotherapy especially in those situations where cure is not possible and where the aim of treatment is to provide good-quality prolongation of life. In these circumstances, measures of quality of life enable an estimate of the balance of benefit and side-effects to be made. Objective response to treatment is measured either as a partial response, which is defined as more than a 50% reduction in the size of the tumour, or complete response, which is a complete disappearance of all detectable disease clinically and radiologically. The terms used to evaluate the responses of tumours are given. Partial remission is often associated with a reduction in symptoms and improvement in quality of life. The rate of regrowth of the tumour is dependent on the underlying doubling time of that particular tumour. For more rapidly growing tumours a partial response may not be associated with very much prolongation of life, whereas with more slow growing tumours responses may continue for a long time. Complete remission is a necessary prerequisite for cure but unfortunately many patients who achieve complete remission will subsequently relapse because of the presence of residual microscopic disease. Many strategies have been developed to try and increase the proportion of patients with complete remission who go on to long-term remission or cure.