Recent research has shown the importance of the patient's point of view on the goals of medical care, and now health-related quality of life (HR-QOL) has become an important endpoint of clinical studies. However, as HR-QOL is essentially a subjective, personal concept determined from the viewpoint of the patient, it is fundamentally important to understand the concept and use the HR-QOL assessment, to express both the subjective and qualitative concept of HR-QOL in an objective and quantitative way that meets the patient's true needs, and also to obtain high-quality information about HR-QOL. In this article, we describe the concept of HR-QOL, the purpose of HR-QOL measurement, the approach to the HR-QOL assessment, instruments used in the measurement of HR-QOL, and general principles of HR-QOL measurements. We also review the current status of HR-QOL assessment of medical treatment in breast cancer.