This study examined factors that contribute to Chinese and United States middle school science teachers' perceptions of autonomy support. Autonomy is one component of self-determination theory and has been associated with intrinsic motivation. The study used a mixed methods design including quantitative data collected through an online survey and qualitative data collected through open-ended interview questions. The online survey consisted of four assessments related to teachers' self-determination, perceptions of constraints at work, perceptions of students' self-determination, and level of autonomy support for students and allowed for the testing of the structural model developed by Pelletier, Seguin-Levesque, and Legault (2002). Exploratory factor analysis (EFA) of responses for the combined teacher sample (n=201) was carried out for each of the survey assessments. Significance testing for Chinese (n=107) and U.S. (n=94) teachers, based on the factors resulting from EFA, revealed significant differences in teachers' self-determination and perceptions of constraints at work. No significant differences were found for teachers' perceptions of students' self-determination or level of autonomy support for students. Multiple regression was used to predict teachers' autonomy support for students. For the Chinese teachers, perceptions of constraints at work, teachers' self-determination, and teachers' perceptions of student motivation were found to significantly predict teachers' autonomy support. For the U.S. teachers, teacher motivation was the only significant predictor of teachers' autonomy support. A sub-sample of the Chinese and U.S. science teachers (n=19) were interviewed about their perceived levels of autonomy support, constraints at work, and their students' self determination. The analyses of the interviews showed that teachers in both countries reported that autonomy was important to their motivation and the quality of instruction they provided to students. Teachers from the two countries differed in their satisfaction with current levels of autonomy and reported different constraints on teaching science related to materials, lab space, curriculum standards, and assessment. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]