There are several large bodies of political science literature that place either central or tangential importance on nonpolitical identities. Implicit in literatures on race, gender, and religion (hereafter “identity” literatures) is the notion that these identities have nontrivial political implications. Similarly, voting behavior scholars have long recognized that a myriad of demographic characteristics and primary and secondary group associations have a significant impact on partisanship, ideology, policy preferences, voting behavior, and political participation. Yet, despite the underpinnings of identity in each of these bodies of literature, I argue that there are several unsettled issues within and between these literatures that can be resolved only through a deductive theory of why and how identity operates to inform political behavior. Some of these issues include heterogeneity within demographic groups leading to differential political behaviors, a disproportionate emphasis on understanding the roots of party identification that has hamstrung the broader study of identity and political engagement, and the evolution of these bodies of literature in relative isolation from one another. In short, the lack of a comprehensive theoretical framework on identity and political engagement has left many theoretical and empirical questions unanswered and under-explored.My dissertation confronts these matters by developing the Identity Theory of Political Engagement (ITPE), which is rooted in three identity propositions that I derived from extant theories in sociology and social psychology. In short, they state that individuals have a salient identity that is hierarchically superior to all other identities, that this salient identity has a subjective meaning that is either group-based, personal, both, or neither, and that the salient identity and its subjective meaning serve as a schema that guides behavior. Borne out of these identity propositions, the ITPE develops a new concept, identity-based political efficacy, which is the idea that individuals believe they can make a difference regarding the political issues facing their salient identity, and that government is responsive to the issues of the salient identity. The ITPE hypothesizes that the subjective meaning of the salient identity and the level of identity-based political efficacy yield four different identity mechanisms of political engagement, each of which shows up in disparate political science literatures: group pressure, self-interest, group interest, and agnosticism. Finally, the ITPE predicts that these mechanisms are process variables that mediate the relationship between the salient identity and political engagement, broadly construed. In sum, the ITPE is a parsimonious theory of how and why identity operates to inform political behavior. It resolves many of the problems that exist within and between the identity and voting behavior literatures, while simultaneously building on and merging insights from these literatures. More specifically, while the ITPE calls into question the essentialization of identity groups, the non-contextualized categorization of individuals, and the disproportionate emphasis on party identification (as opposed to other identities in understanding political behavior), it also amply credits these literatures with providing the requisite foundational knowledge on which to build. To test this theory, I designed a unique survey and administered it to three sample populations, including a matched representative sample of 400 Americans. The goals of the survey were to operationalize new concepts, such as identity-based political efficacy and the mechanisms of political engagement, and develop a single survey instrument capable of testing the central hypotheses of the ITPE. In general, I find very strong support for the ITPE. First, I find that individuals do have a salient identity, which is subjectively defined as group-based, personal, both, or neither. Second, I find that the operationalization of identity-based political efficacy is valid, reliable, and distinct from traditional measures of political efficacy. Most importantly, the subjective meaning of the salient identity and varying levels of identity-based political efficacy do accurately predict the four identity mechanisms of political engagement. These mechanisms, in turn, largely determine broader levels and types of individual political engagement. Lastly, the mechanisms often mediate the relationship between the salient identity and political engagement. These results illustrate that in order to more fully understand the motivational factors behind political engagement, it is important to expand conventional understandings of identity and political efficacy.