Regular participation in physical activity has been linked to improved health status in children and adults and is one way to fight obesity. Many researchers and professional organizations identify physical education as a potentially important component in efforts to improve children's health, but physical education classes alone are not sufficient to meet the recommended physical activity requirements for children. From this perspective, it is important to investigate ways to design physical education class environments that will facilitate the adoption of physically active lifestyles, and that was the purpose of this study. Specifically, the relationships among key motivational constructs and engagement variables were investigated using a structural equation model. The model was generated based on self-determination theory. It was hypothesized that a pattern of positive relationships would exist among perceptions of the mastery climate, levels of self-determination, and attitudes, and that those three variables would be predictors of perceived competence. Perceived competence, in turn, was put forth as a direct predictor of the outcome variable of engagement. Participants were 401 middle school physical education students enrolled in two public schools. Students completed a series of surveys to measure the following motivational constructs: self-determination (SIMS, Guay et al. 2000), perceived motivation climate (LAPOPECQ, Papaioannou, 1994), attitude toward physical education (Subramaniam & Silverman, 2000), and perceived competence (CY-PSPP, Whitehead, 1995). Engagement in physical activity was assessed using three data sources: a self-report questionnaire (PAQ-C, Crocker et al. 1997), intention to be active (Standage et al. 2003) and pedometer step counts during physical education classes. Data were collected during regular physical education classes during six instructional periods. Maximum likelihood was used to estimate the model. Fit indices for the model approached acceptable limits (Stevens, 2002). Forty-two percent of the outcome variable engagement was explained by the model. Consistent with theoretical predictions, there was a pattern of positive relationships among higher levels of self-determination, perception of a mastery climate, and positive attitudes. The link between perceived competence and engagement was significant, as predicted. Perception of a mastery climate had a significant an indirect effect on engagement through perceived competence. The results of this study provide important insight into the factors that lead to active engagement in physical education classes. Findings support the notion that providing a mastery climate and structuring the environment so that students can be successful in their physical education endeavors foster active engagement and the intention to engage in physical activity.Keyword(s): exercise/fitness/physical activity, physical education PK-12, research