Over the past decade and a half, a number of psychologists and behavioral scientists have felt the need for a method that allows the study of the ongoing everyday behavior and the experiences of people in their normal life and environments. For this purpose, investigators have sampled thoughts and experiences at random points in time over several days in a person's life. Typically, subjects in these investigations carry electronic beeping devices and report or rate their experiences by filling out questionnaires at the time of a signal. This methodological need arose from different sources, for instance, the call for ecological validity of behaviors studied, the attempt to understand behavior as being embedded in an ongoing context and sequence, the increased interest in the interaction of situation and person variables, the study of situations as individuals seek them out, the necessity to generalize from laboratory settings to the real world, the wish for extensive study of single individuals, or the attempt to study processes that are difficult to create in a laboratory setting. Developments toward the ESM originated in one form or another in the 1960s as behavioral observation gained a foothold in psychology and medicine. Several researchers often simultaneously in different places around the globe, and without, until recently, being aware of (or citing) each other's work have developed this approach (DeVries, 1987; Hormuth, 1990; Csikszentmihalyi & Larson, 1987; Hurlburt, 1987a).