Quality of Life (QOL) is a generic term covering a wide variety of end points. It generally refers to a multitude of subjective experiences important to people's lives. Four domains contribute to this overall effect: physical and occupational function, psychological state, social interaction, and economic status/factors. Quality of life has emerged as an important outcome measure of optimal medical care, particularly for the treatment of chronic conditions. Four types of clinical trials in which QOL should be evaluated are: (1) the intervention has an effect on symptoms but not mortality or complication rates; (2) the intervention causes a high frequency of side effects; (3) prevention trials, and (4) trials designed to lower cost or rate of adverse effects. Standards for such trials are currently being developed. The one area of current agreement is the need for the use of validated scales of measurement. The use of both a general scale and a disease- (or age group) specific scale may provide the best assessment of overall QOL.