This chapter provides an introduction to the assessment of the quality of life (QoL) of cancer patients. It considers how QoL is defined and how it can be measured using either generic or cancer-specific QoL questionnaires (e.g., the European Organization for Research and Treatment of Cancer QLQ-C30, Functional Assessment of Cancer Therapy General Questionnaire, and the Edmonton Symptom Assessment Scale (ESAS) in palliative care). Also described briefly is the use of computer adaptive testing (CAT)). Examples are provided of how QoL can be assessed in the context of clinical oncology research (both observational and evaluative QoL studies) and how QoL assessment can be integrated into daily clinical oncology practice. Special attention is paid to the ways in which QoL data can be interpreted and to establishing the clinical significance of QoL results. Finally, the chapter outlines some of the future directions for QoL research in the oncology setting.