This study examined the utility of motivation as advanced by self-determination theory in predicting objectively assessed bouts of moderateintensity exercise behavior. Participants provided data pertaining to their exercise motivation. One week later, participants wore a combined accelerometer and heart rate monitor (Actiheart; Cambridge Neurotechnology Ltd) and 24-hr energy expenditure was estimated for 7 days. After controlling for gender and a combined marker of BMI and waist circumference, results showed autonomous motivation to positively predict moderate-intensity exercise bouts of ≥10 min, ≥20 min, and an accumulation needed to meet public health recommendations for moderateintensity activity (i.e., ACSM/AHA guidelines). The present findings add bouts of objectively assessed exercise behavior to the growing body of literature that documents the adaptive consequences of engaging in exercise for autonomous reasons. Implications for practice and future work are discussed.Keywords: exercise psychology, health behavior, physical activity, health A compelling body of research documents the numerous physiological and psychological health benefits associated with regular physical activity and/or exercise participation (cf. American College of Sports Medicine [ACSM], 2006). 1 In a recent physical activity and public health recommendation, the ACSM and the American Heart Association (AHA) recommended that to promote and maintain such health benefits, adults should accumulate 30 min or more of moderate-intensity activity on a minimum of 5 days each week (ACSM/AHA guidelines; Haskell et al., 2007). 2 Despite the known health benefits of moderate-intensity activity, the World Health Organization (WHO, 2003) estimates that over 60% of the world's population is insufficiently active to profit from regular exercise and/or physical activity. Therefore from a public health perspective, understanding the determinants of moderate-intensity exercise represents an important avenue of research. Representing why an individual is moved to act , an individual's 338 Standage, Sebire, and Loney underlying motivation toward expending the energy and effort required to partake in prolonged bouts of moderate-intensity exercise would appear to represent an important antecedent to such behavioral engagement.While numerous theoretical models of motivation have been advanced to account for exercise behavior, a growing number of empirical studies have employed self-determination theory (SDT; see Hagger & Chatzisarantis, 2007, for exercise-specific reviews). An appealing feature of SDT is that motivation is considered from a multidimensional perspective, distinguishing between autonomous and controlled types of motivational regulation and their differential impact on an individual's psychological well-being, behavioral quality, persistence, functioning, and learning .When autonomously motivated, individuals endorse their own actions, acting with a full sense of volition because they find the activity to hold inherent interest and/or perso...