In measuring the characteristics of mass-communication audiences, three dimensions are frequently used: demographics, social class, and psychological characteristics. The author proposes a fourth, "life style," which offers reliable data, is more colorful than demographics and more useful than psychological characteristics. This dimension is designed to answer questions about people in terms of their activities, opinions, and interests (AOI's). To date, data for these studies has come from adults who are members of national mail panels. Information on AOI's, demographics, media usage, media preferences, and product or brand usage is obtained. The data is then analyzed in one of two ways. Correlational analysis indicates the difference in life styles between users and nonusers of a product or readers of one magazine versus those of another magazine. Cluster analysis divides the sample by life style and relates those life styles to product use or media preference. Examples from previous life style studies illustrate the two techniques. The life style dimension has been of use in advertising and marketing and should be useful in mass communication research. (JK)