Extremism in society is the source of enormous human suffering and represents a significant social problem. In this article, we make a case for the urgency of understanding the psychology of societal extremism, discuss the diverse forms that extremism can take, and identify uncertainty as a correlate of and quite possibly precondition for extremism. We discuss the concept of uncertainty and the burgeoning social psychological research on uncertainty and its links with various forms of extremism. Thus, this article frames and contextualizes the subsequent articles in this issue of the Journal of Social Issues, on the psychology of the relationship, and possible causal link, between uncertainty and societal extremism.