Experience Sampling (ES) is a new psychiatric assessment method that provides a representative sample of moments in a person's daily life. It uses a beeper to signal subjects to fill out self-reports at preselected but randomized time points, thus providing information about an individual's mental status or symptoms within the context and flow of experience. The method is used like the Holter monitor in cardiology, to assess functioning outside of the laboratory. Experience Sampling thereby avoides the problems of global and retrospective recall that often limits psychiatric research. The opal of ES research in Maastricht has been to clarify the temporal and social context of symptom fluctuations for use in clinical care and for the improvement of psychiatric assessment and treatment. In this article, procedural, methodological, and treatment experience sampling applications are introduced to a Dutch psychiatric audience. Patient samples are drawn from chronic mental patients, anxiety, pain, and normal subjects. The studies demonstrate the descriptive power and clinical utility of experience sampling. The data illustrate that illness is actually a variable experience and not a static property of the individual.