A key characteristic of democratic politics is competition between groups, first of all political parties. Yet, the unavoidably partisan nature of political conflict has had too little influence on scholarship on political psychology. Despite more than 50 years of research on political parties and citizens, we continue to lack a systematic understanding of when and how political parties influence public opinion. We suggest that alternative approaches to political parties and public opinion can be best reconciled and examined through a richer theoretical perspective grounded in motivated reasoning theory. Clearly, parties shape citizens' opinions by mobilizing, influencing, and structuring choices among political alternatives. But the answer to when and how parties influence citizens' reasoning and political opinions depends on an interaction between citizens' motivations, effort, and information generated from the political environment (particularly through competition between parties). The contribution of motivated reasoning, as we describe it, is to provide a coherent theoretical framework for understanding partisan influence on citizens' political opinions. We review recent empirical work consistent with this framework. We also point out puzzles ripe for future research and discuss how partisan-motivated reasoning provides a useful point of departure for such work.