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Insufficient physical activity is one of the ten leading global risk factors for mortality (World Health Organization, 2009), with worldwide prevalence ranging from 16.6 to 34.5% for adults and between 77.5 and 84.0% for school-age adolescents (World Health Organization Global Health Observatory, 2010). These estimates are based on self-reports and are, therefore, likely underestimating the degree of inactivity. In this context,“insufficient” is defined as less physical activity than recommended in evidence-based guidelines. For example, healthy adults are advised to accumulate (in bouts of at least 10 min) a minimum of 150 min of moderate-intensity aerobic physical activity per week (or at least 75 min of vigorous-intensity aerobic physical activity or a combination of moderate and vigorous aerobic physical activity), in addition to performing muscle-strengthening activities on two or more days (World Health Organization, 2010). Exercise is defined as “a subset of physical activity that is planned, structured, and repetitive and has as a final or intermediate objective the improvement or maintenance of physical fitness”(Caspersen, Powell, & Christenson, 1985, p. 126). Exercise represents an efficient way of avoiding the health disadvantages of physical inactivity. However, most people fail to perform regular exercise. This article presents a theory that is intended to explain why people in a state of physical inactivity do or do not initiate action for exercise. The theory focuses on the psychological processes that govern momentary behavior and deals specifically with exercise-related feelings; it emphasizes the concept of situated decisions about exercise …