K-12 online education has provided new opportunities for students to learn and succeed. Although research is still in its infancy, studies in K-12 online learning suggest that the teacher plays a critical role in whether students succeed or fail. This paper provides an introduction to why professional development is so critical to online education. Research is presented that describes when professional development succeeds, and when it fails. Specific recommendations are included for the larger field of K-12 online education, virtual schools, and K-12 online instructors.