This dissertation presents the findings of an exploratory, multiple-case study in which eight public secondary school teachers engaged in a professional development (PD) program centered on improving teacher instruction for English learners (ELs). The participants worked as co-teachers with teacher credential candidates, serving as both mentors and collaborators in professional development. The PD program focused on improving participants' instructional practices with ELs as a way to support the high numbers of ELs enrolled in California public schools. The PD included cycles of inquiry, collaborative dialogue within a professional learning community, working with a university consultant, ongoing reflection, and collaborating with a teacher candidate as a way to improve their own teaching practices. This study explores changes in mentor teachers' beliefs, practices and self-efficacy toward working with ELs over the course of a seven-month professional development project. This research was viewed through the framework of "Mezirow's Transformative Learning Theory." Findings suggest that all participants entered the PD experience with positive beliefs toward the academic capabilities of ELs and the desire to improve their own instructional practices. Data from this research reflects the need for PD programs to include differentiated, or individualized, activities that help to reach the professional growth needs of participants who are in various stages of transformative learning. [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.]