There is a growing need for qualified online instructors to teach the expanding population of online K-12 students. To meet this need, teachers must be provided learning opportunities to acquire the specific types of knowledge and skills necessary to teach online. In this systematic review of the literature, we utilize the TPACK framework to aggregate the types of knowledge and skills required to teach online and examine both the extent to which these elements are addressed in existing programs and are based on empirical research. Findings suggest that the types of knowledge and skills based on empirical research originate from few studies and that most programs address only a small subset of knowledge and skills, varying greatly without uniformity in content or learning experiences.