This report presents a summary of empirical studies of K–12 online and blended instructional approaches that offer differentiated learning options. In these approaches, instruction is provided in whole or in part online. This report includes studies that examine student achievement outcomes and summarizes the methodology, measures, and findings used in the studies of these instructional approaches. Of the 162 studies that were reviewed, 17 met all inclusion criteria and are summarized in this report. The majority of the studies examined blended instructional approaches, while all approaches provided some means to differentiate the content, difficulty level, and/or pacing of the online content. Among the blended instructional approaches, 45 percent were designed to support differentiation of the in-class component of instruction. The majority of studies examining these approaches compared student performance on common standardized achievement measures between students receiving the instructional approach and those in comparison classrooms or schools. Among the most rigorous studies, statistically significant positive effects were found for four blended instructional approaches.