Blended learning environments that merge learning strategies, resources, and modes have been implemented in higher education settings for nearly two decades, and research has identified many positive effects. More recently, K-12 traditional and charter schools have begun to experiment with blended learning, but to date, research on the effects of and issues related to blending in these settings has been limited, and the results less clear. In this article, we draw from relevant published research, reports, and book chapters from higher education and K-12 to discuss what K-12 teachers should know about blended learning environments. This review suggests that effective teacher preparation for blended instruction must integrate three broad components--"contextual," "instructional," and "technological"--each of which is closely aligned with common instructional design processes familiar to most teachers.