In recent years, there has been increasing interest in incorporating the concept of "Quality of Life" in the evaluation of medical interventions. The lack of a general definition which can function as a basis for the scientific evaluation of medical treatment is, however, a problem. The definition proposed in this paper contains three main factors: Subjective well-being, Health and Welfare. "Subjective well-being" is the central component in the evaluation of Quality of Life, referring to the individual's perception of his life situation, "Health" is a subjective as well as objective evaluation of physical and mental status, while "Welfare" reflects the objective environmental factors. Thus, the evaluation of Quality of Life in medicine should ideally address these three main factors. Although the importance of any particular component may vary, subjective well-being and health should always be included.