This study reports the first experimental estimates of the longitudinal effects of exposure to fact-checking. We also conduct a comprehensive panel study of attitudes toward fact-checking and how they change during a campaign. Our results are generally encouraging. The public has very positive views of fact-checking and, when randomly exposed to it, comes to view the format even more favorably. Moreover, randomized exposure to fact-checks helps people become better informed, substantially increasing knowledge of the issues under discussion. We also document several important challenges facing fact-checkers, however. Most notably, interest in the format is skewed towards more educated and informed members of the public. Republicans also have less favorable views of the practice than Democrats. Continued growth of the medium will depend on broadening its appeal to these groups.