The role of the news media in defining the important issues of the day, also known as the agenda-setting influence of mass communication, has received widespread attention over the past 20 years. Since the publication of McCombs and Shaw's seminal empirical study, more than one hundred journal articles and monographs have appeared. This collection exemplifies the major phases of research on agenda-setting: tests of the basic hypothesis, contingent conditions affecting the strength of this influence, the natural history of public issues, mass media influence on public policy, and the role of external sources from the president to public relations staffs on the news agenda.