This paper overviews the problem of physical inactivity from the perspective of exercise adherence. Five conundrums are described that have retarded advances in knowledge about the causal determinants of physical activity and successful internenlions that increase long-term physical activity and exercise adherence: (a) The Revolving Door (i.e., adoption vs. maintenance); (b) Marketing Change: What is the Message ? (e.g., social marketing vs. product marketing; (c) Blind Intervention (i.e., what are the mediators of physical activity?); (d) Premature Theory (e.g., the fallacy of linear modeling and the promise of structural covariance modeling); and (e)The Missing Dependent Vdriahle (i.e., defining and measuring physical activity).