The belief that life stress influences illness in patients was discussed. Although indirect evidence from controlled laboratory studies and from extreme situations provides strong indications of the general relationship between stress and illness, the evidence concerning the effects of naturally occurring and more ordinary stressful life events is less clear. Although research indicates that life events are associated with a wide range of disorders and distress, it does not provide a clear picture of the nature and strength of this relationship. To clarify how stressful life events affect health and illness, methodological issues that have emerged from research to date must be considered, and a major substantive problem that has been largely neglected must be tackled. The methodological issues include definition of the populations of life events to be studied; measurement of the magnitudes of the life events; and use of a research design appropriate to the question to be answered. The substantive issue concerns factors that mediate the impact of life events.