Agenda-setting theory, as originally formulated in 1972 by Maxwell McCombs and Donald Shaw, explains the relationships between the emphasis that the mass media place on issues and the importance that media audiences attribute to those issues. While agenda-setting theory started out as an explanation of media impact on political behavior and attitudes during election years— specifically, the ways that news media coverage can prioritize issues, or set the agenda, for the public—in the decades since McCombs and Shaw’s initial study was published, the theory has inspired hundreds of subsequent explorations into the ways that media and other institutions prime and frame issues and events for their audiences and therefore influence and shape public opinion, either intentionally or unintentionally. As a result, agenda-setting theory has had a profound influence, not only on mass communication and political communication research, but also on the development of various organizational communication, persuasion, and diffusion-of-innovations theories. At the same time, the original theory has been revised by Maxwell McCombs, one of its codevelopers, in ways that expand and even contradict one of its key tenets.

Source : Littlejohn, Stephen W and Karen A.Floss. (2009). Encyclopedia of Communication Theory.USA:SAGE. 100