Thesis (Ph.D., Curriculum & Instruction) | This case study investigates the perspectives of four Taiwanese English ESL (English as a second language) teachers with regard to their participation in an online professional development course. To build a collaborative online professional development community, in which teachers acquire professional knowledge to improve teaching instruction and student learning, the study addresses teachers' voluntary participation and intrinsic motivation to engage in self-improvement and continuous learning (Maor, 2003; Yeh, 2007). Moreover, the research suggests that plans for professional development activities must include a bottom-up design (Guskey, 2000), a means of practical implementation (Darling-Hammond, 2005), and be based on a teacher-focused model (Yang & Liu, 2004). Data sources include interviews, pre- and post-questionnaires, electronic documentation of subjects' participation, and field notes. Triangulation is used to determine themes and concerns, and findings are presented through rich, thick description. Although the teachers sometimes faced challenges when they applied the new teaching activities and/or materials from the online professional development course to their classrooms, the overall results reveal that they found the course beneficial with regard to not only new knowledge and skills but also in terms of their positive feelings of engagement in their work. Further the study indicates that an online professional development course (a) can offer teachers a learning opportunity that is not geographically or temporally limiting; (b) may facilitate the transfer and construction of pedagogical knowledge and professional skills that can have implications for teachers' practice, and consequently on students' learning; and (c) can open up opportunities for international collaboration and social interaction with others.