The present dissertation is based on three studies carried out with a single dataset, each addressing different aspects of an Adult ESL online professional development (OPD) course and its impact on current and prospective teachers' professional knowledge as well as their beliefs and attitudes about adult English language teaching and learning. Research subjects were seventy-six prospective and current Adult ESL educators who enrolled in a facilitated, asynchronous, eight-week, fully online course offered for a fee through a community college in the Southeastern United States. The first study provides a portrait of Adult ESL OPD participants at the start of the course, the factors that influenced their engagement and performance during the course, and their persistence and satisfaction upon completion. Unlike past studies of Adult ESL OPD citing low technology skills and high dropout rates among Adult ESL educators, neither of these factors posed a significant barrier to full participation in this course. Motivation and Adult ESL teaching background had the stronger influence instead.

The second study used a pre- and post-test design to delve into professional knowledge as an important dimension of teacher change. A two-way ANOVA indicated that current and prospective teachers' test scores were comparable but course activities associated with each group's scores differed significantly. Hierarchical regression analyses revealed that controlling for pre-test scores, participants' total online discussions postings were the strongest predictor of post-test scores. This relationship was noticeably stronger for prospective Adult ESL educators whose discussion participation served as an important conduit for their professional learning during the course. In the final article, a mixed methods approach was applied in order to examine relationships between journal writing and change in beliefs and attitudes. Journal word counts were not associated with participant change, but levels of reflection expressed through journal writing were. Illustrative case studies of the subsample suggested that these varying reflection levels may have been influenced by participants' previous teaching backgrounds, specific reasons for taking the OPD, and experiences in the field of Adult ESL. Sample size is noted as a primary limitation in the three studies.