Physical education is important to promote physical activity of adolescents with and without disabilities, but many adolescents are not active during physical education classes. Innovative instructional strategies are imperative to change this phenomenon, but it will be challenging to develop effective instructional strategies without thorough understanding of students' physical activity behavior in physical education settings. Two studies were conducted to comprehensively understand physical activity behavior of adolescents with and without disabilities in inclusive physical education classes at middle schools. The first study investigated the utility of the integrative theory to predict students' physical activity intentions and behavior at the intrapersonal level. A total of 577 participants, including 24 adolescents' with disabilities, were recruited from 8 middle schools in Korea. In a prospective design, participants' psychosocial constructs and physical activity data were collected by survey questionnaires and electronic pedometers. A multilevel (design-based) structural equation modeling using maximum likelihood estimation with robust standard error correction found that students' attitudes, subjective norms, and barrier-efficacy significantly predicted students' goal intentions. Students' implementation intentions and task-efficacy were significant predictors of physical activity behavior. In addition, implementation intentions completely mediated the relationship between goal intentions and physical activity behavior. The second study investigated three conceptual models, including process-product model, student mediation model, and ecological model, to predict students' physical activity behavior at the interpersonal and environmental levels. A total of 13 physical educators teaching inclusive physical education and their 503 students, including 22 students with disabilities, were recruited from 8 middle schools in Korea. A series of multilevel (model-based) regressions with maximum likelihood estimation showed that the ecological model was the most effective model in prediction of students' physical activity behavior. Specifically, it was found that teachers' teaching behavior and students' implementation intentions were significant predictors of the students' physical activity behavior when interacted with gender, disability, lesson contents, instructional models, and class locations. In conclusion, findings suggest that intrapersonal, interpersonal, and environmental predictors provide a systematic account in the understanding of students' physical activity behavior in physical education settings. Future studies should consider all three factors simultaneously to effectively develop instructional strategies that promote physical activity of adolescents' with and without disabilities in physical education classes. / Graduation date: 2013