Communication theories detail the reactions that follow from actions, allowing those who understand them to explain and even predict the consequences of certain patterns of communicating. Theories that predict how audiences will react in certain situation or to certain messages and how they are presented are especially helpful to public relations practitioners who hope to achieve persuasion and information objectives with every message constructed. Ultimately and most simply, theories provide practitioners with a schematic that guides message design so that communications have the best chance of achieving the goals outlined in a public relations program. Anyone without a basic knowledge of how to predict behavior following communication is essentially "shooting in the dark," communicating somewhat randomly with an audience and hoping for a positive outcome.

While there are many theories that can be applied to public relations, those that are perhaps most useful fall into one of three categories: relationship theories; cognition and behavior theories; and general communication theories. First, relationship theories explain how entities interact. These are useful because they not only outline how public relations fits into the organizational structure but how stakeholders fit into the mix of publics upon whom practitioners focus. Second, theories of cognition and behavior help explain how audiences assimilate information and how they then act in its response. The public relations value in these theories centers on the fact that they make it possible to anticipate how an audience will respond to various messages and, knowing this, how a practitioner can nudge publics to perform in a manner that is most positive toward the client. Third, general theories of mass communication outline the reasons that the public attends to messages and how the consumption of mediated information influences perceptions of reality. These theories are particularly useful to public relations practitioners due to the fact that the mass media often serves as a conduit through which organizational communication passes, and any understanding of how information reaches and ultimately affects the public helps guide PR activities.

Finally, the discussion of theories gives way to the delineation of models of public relations. Because public relations involves a complicated mix of activities intended to reach different publics through different means and with different goals, practitioners seldom serve in static capacities. An understanding of the models helps provide budding practitioners with an understanding of the different roles that they will be asked to fill in the professional world and will help outline actions that should be taken when acting in one capacity or another.