This essay by a pioneer scholar in the field broadly summarizes the collective body of findings from hundreds of agenda-setting studies of the past 20 years and suggests fruitful research lines for the future. McCombs finds that, as with pealing a sweet onion, there are layers of research, each with its distinct tantalizing aroma of conclusions. Journalism practitioners, scholars, students and scholars from political science and other disciplines have contributed many perspectives within the context of a variety of data-gathering techniques and subjects. Broadly speaking, he finds that scholars tend to be those who carefully survey and mark ground that has already been discovered but only loosely explored and those who are tempted to move beyond the boundaries of the known. As the circle of research activity enlarges, we know much. But there is more that we do not know, and that is more exciting.