This chapter reviews major advances in our understanding of the ways mass political communication influences citizens’ information processing, reactions to threats and opportunities, issue positions, and vote choice on Election Day. We focus on the conditions under which exposure to political communication leads to attitudinal and behavioral change. Once assumed to be a simple process of exposure and response, the effects of political communication, we now know, are often subtle and dependent on characteristics of the source, the medium, the message, and the motivations and abilities of the viewer. As our understanding of human psychology grows, so does our ability to predict when effects of mass media in the political domain will become large and consequential.