Effective online teaching requires instructors to develop knowledge and new skills conducive to creating meaningful and successful learning experiences for online students. This exploratory case study examined 34 prospective online instructors' reflections on their development of online teaching expertise at the end of a graduate course. We analyzed participants' written reflections for their developed understanding of online teaching. We presented and discussed three major themes: (1) Perceptions of online instructors' roles; (2) Development of pedagogical and technological knowledge; (3) Shifting from online learners' perspectives to online instructors' perspectives. These themes represented the major patterns revealed in the prospective online instructors' reflections, showing the areas of expertise they had developed in the class. We also provided implications for programs to develop online instructors and directions for future research.