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. v ACKNOWLEDGEMENTS My favorite movie character, Marty McFly, from my favorite childhood movie, Back to the Future, once said, "If you put your mind to it, you can accomplish anything." This idea was one that I have always found to be true throughout various points in my life. Never has this idea been truer than with the process of writing this dissertation. There are multiple people that must be acknowledged and thanked for being sources of strength, encouragement, inspiration, and/or support for my learning efforts. The first person that I must thank is my wife, Dr. Elissa Cruz: Your love and support during my journey throughout graduate school helped to carry me through, especially during the toughest times. To my daughters, Jessalyn, Kalliana, and Mariella: You girls were my source of inspiration. Thank you for giving me your unconditional love, even during the times when I had to work late or spend the weekend writing. I enjoyed taking breaks from school and work to attend softball games, coach soccer practice, attend dance recitals, participate in art classes, and help out with various school events. Being your dad continues to be the greatest privilege of my life. I look forward to supporting your life goals and helping you all to pursue your dreams. You three make your mom and I happy and proud every day. To my father-in-law and mother-in-law, John and Emma Cruz: I cannot count the number of times that you both cooked for our family and took care of the kids while I was in class, finishing a class assignment, or working on my research...