Motivation is an important topic of concern for teachers. A review of motivation research, though, revealed that students have been the focus of motivation research. When teachers have been included in studies, researchers have asked teachers to evaluate specific students, compared teacher and student perception of the same phenomena, or asked teachers to respond to their own "a priori" frameworks. Research on lay theories and teacher beliefs also yielded ideas important for this study. The purpose of this study was to explore teacher thinking about student motivation among middle school teachers. I designed a multi-method qualitative study to investigate teacher thinking about student motivation at the middle school level. In the first phase, I distributed a five-item, open-ended written questionnaire to teachers in two middle schools. The second phase of the study was a multiple case study of four teachers. Through interview and observation, I investigated each teacher's lay theory of motivation. Constant comparative analysis and a coding framework grounded in motivation research were used to analyze data. Teachers, I found, tend to define motivation in terms of expectancy. At the same time, they also tend to prioritize belonging over value and expectancy. These findings show that teachers, as a group, consider each domain to be important, and they understand the domains to interact and influence each other. Implications for teacher education, policy, and future research are discussed. [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.]