Misinformation occurs when people hold incorrect factual beliefs and do so confidently. The problem, first conceptualized by Kuklinski and colleagues in 2000, plagues political systems and is exceedingly difficult to correct. In this review, we assess the empirical literature on political misinformation in the United States and consider what scholars have learned since the publication of that early study. We conclude that research on this topic has developed unevenly. Over time, scholars have elaborated on the psychological origins of political misinformation, and this work has cumulated in a productive way. By contrast, although there is an extensive body of research on how to correct misinformation, this literature is less coherent in its recommendations. Finally, a nascent line of research asks whether people's reports of their factual beliefs are genuine or are instead a form of partisan cheerleading. Overall, scholarly research on political misinformation illustrates the many challenges inherent in representative democracy.