Quality of life is a subjective multidimentional concept that can be assessed by means of validated questionnaires completed by patients. The psychological effects of a diagnosis of cancer and the physical effects of the disease and its treating have a major impact on a patient's health-related quality of life. Much cancer treatment, especially chemotherapy for metastatic disease, is given for palliation. Palliation implies improvement in either the duration or quality of life remaining. However, treatment patients with common metastatic tumours to prolong life is generally unsuccessful, so improving quality of life is a more realistic goal. Most trials involve evaluating shrinkage of a tumour (i.e., tumour response), which does not imply a benefit to the patient. Few trials have assessed quality of life directly, although several validated instruments, described here, are available to quantify quality of life in cancer patients. These instruments represent a wide scope, from evaluating general health to assessing the quality of life of patients with specific types and stages of cancer. They respond to changes in clinical state and are strongly predictive of survival. Measures of quality of life should be incorporated in all clinical trials where treatment is palliative, and a simple, relevant measure of quality of life should be used as a (or the) primary outcome measure. Other measures of quality of life are important to ensure that gains in one area do not occur at the expense of others. A few large trials incorporating these principles have shown that chemotherapy can provide palliation for patients with advanced cancer.