The number of English language learners (ELLs) in the United States school system is growing rapidly. Much of the responsibility for teaching ELLs lies with regular classroom teachers. However, little training is being provided to help them. From a sociocultural perspective and drawing on constructive-developmental theories of adult learning and development, this study explores the potential for transformative learning in a one semester online English as a Second Language for Educators course. It is argued that if a single course is all that is required of teachers, the goal must be "transformative learning," defined as a change in "how" a person knows rather than just "what" a person knows. The research questions were: (1) How did teacher candidates experience the online ESL for Educators course, and what roles did their background and prior experiences play? (2) What shifts in thinking took place in their understandings about working with culturally and linguistically diverse learners as a result of their participation in the course? (3) Which course activities, according to the teacher candidates, contributed to transformational shifts in thinking, and what role did the online learning environment play? Drawing from both qualitative and quantitative data, the study describes in depth the experiences of six adult learners (four females and two males ranging in age from late 20's to early 50's), including their backgrounds, prior experiences, teaching context and life circumstances during the time of the study, reported changes in understandings about linguistic diversity based on course participation, and epistemological tendencies (sources of authority, senses of self, ways of knowing). The data revealed evidence of shifts in thinking about the education of ELLs, which often emerged as a result of their participation in the field experiences. However, the results also suggest that this particular learning context was not ideal for fostering development and transformational learning. This study calls into question the reasonableness of expecting a one semester online course such as this to adequately prepare educators to work effectively with ELLs. Issues for course and program revision are explored. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]