There continues to be an abundance of research conducted and published about online learning in K-12 settings. In this systematic review, 386 articles on K-12 online teaching and learning published between 2000 and 2019 were reviewed for research themes, participants, school type, research methodologies, and data collection methods. The data was analyzed by the two decades 2000 to 2009 and 2010 to 2019 and showed significant increase in the second decade. There was an increased growth in the number of publications until 2016 after which it stabilized. About 60% of the studies were published in the United States. Areas that are most and less frequently researched are reported and discussed. The article provides implications and recommendations for future research for online learning in K-12 settings.