Influence theory is a systematic study of formal models of the communicative influence of one person or group of people on another person or group. In that sense influence theory is an overarching philosophical discipline that includes aspects of decision theory and game theory as sub-disciplines as well as established models of de facto segregation, cultural change, opinion polarization, and epistemic networks. What we offer here is a structured outline of formal results that have been scattered across a range of disciplinary contexts from mathematics, physics and computer science to economics and political science, supplemented with a number of new models, emphasizing their place within the philosophical framework of a general theory of influence. What such an outline offers, we propose, is the prospect of new and important cross-fertilizations and expansions in formal attempts to model the diverse patterns of communicative influence.