For more than two decades political scientists have discussed rising elite polarization in the United States, but the study of mass polarization did not receive comparable attention until fairly recently. This article surveys the literature on mass polarization. It begins with a discussion of the concept of polarization, then moves to a critical consideration of different kinds of evidence that have been used to study polarization, concluding that much of the evidence presents problems of inference that render conclusions problematic. The most direct evidence—citizens’ positions on public policy issues—shows little or no indication of increased mass polarization over the past two to three decades. Party sorting—an increased correlation between policy views and partisan identiﬁcation—clearly has occurred, although the extent has sometimes been exaggerated. Geographic polarization—the hypothesized tendency of like-minded people to cluster together—remains an open question. To date, there is no conclusive evidence that elite polarization has stimulated voters to polarize, on the one hand, or withdraw from politics, on the other.