In the Nordic countries, working hour reductions have been introduced as a possible occupational health intervention. We evaluated the effects of a working hour reduction from 37 to 30 h a week among 100 employees within older people’s care. The outcome evaluation was broad, including health check-ups and extensive questionnaires covering work climate, work-home balance, health-related measures, well-being, and health behaviors. The results showed no improvements during the 1-year project period, which raises the question whether the results are due to failure in implementation or theory. This chapter asks what is needed for considering a failure as a theory failure, that is, a default program theory in that the activities fail to bring the desired effects. It is suggested that the case described here may be one such example. In order to facilitate the separation of theory failure from implementation failure, we underscore the importance of program theory, also called theory of change or logic models. By scrutinizing the program theory before implementing an intervention, a critical evaluation of the mechanisms linking the intervention to various outcomes can be made. Also, the program theory is helpful for forming testable hypothesis that allows variation in process factors to be tested. ©