Despite well-intentioned professional development programs designed to improve teachers' effectiveness in the use of instructional practices that have the potential to enhance student achievement (e.g., differentiated instruction), a discrepancy exists between the promise these practices hold and their actual use by teachers in classrooms. To address this discrepancy, this study explored how teachers actually learned the beliefs and mental models that accompanied their development of proficiency in the use of differentiated instruction. The conceptual framework for the study was the TRIO model of adult learning. The particular area of focus was the relationship between (a) key experiences, (b) changes in teachers' beliefs, and (c) proficiency in the use of differentiated instruction. The study used an interpretive qualitative design. Semi-structured interviews, intended to identify teacher beliefs and their relationship to key experiences, were conducted with six key participants who were highly skilled in using differentiated instruction to meet the needs of individual students in mixed-ability classrooms in a suburban elementary school. Data were also obtained from classroom observations of these participants prior to the interviews to ascertain that they were, in fact, differentiating their instruction. The interview data were analyzed to identify (a) teachers' key learning experiences, (b) related changes in teachers' beliefs about teaching and learning, and (c) the development of teachers' proficiency in applying differentiated instruction. Findings indicated that the key experiences that shifted teachers' mental models in relation to learning and teaching were (a) collaborating with peers, (b) co-teaching, and (c) working within a coaching model of professional development. Findings also demonstrated that practice in implementing a social curriculum, specifically Responsive Classroom, was a key experience that helped teachers understand their students as learners. In turn, this knowledge about their students enhanced their ability to use differentiated instruction. Finally, all teachers reported that having a personal or professional understanding of students with special learning needs shifted their thinking about what students could or could not achieve. The discussion includes implications for practices that hold promise for increasing teachers' proficiency in meeting the learning needs of individual students in mixed-ability classrooms. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]