Public relations practitioners have long attempted to persuade publics through mass media campaigns. Social scientists, likewise, have searched for the most persuasive mix of messages and media. Most of this research, however, indicates that the media seldom change attitudes and behavior, and that persuasion is, therefore, an unrealistic objective for a media campaign. Led by Maxwell McCombs, the author of this article, communication researchers have discovered a logical and more realistic effect of media: While the media do not tell people what to think, they tell people what to think about. That is, the media determine which issues—and which organizations—will be put on the public agenda for discussion. In this article, McCombs summarizes research on agenda-setting and then discusses its implications for public relations.