Transparency of research is a large concern in political science, and the practice of publishing links to datasets and other online resources is one of the main methods by which political scientists promote transparency. But the method cannot work if the links don’t, and very often, they don’t. We show that most of the URLs ever published in the American Political Science Review no longer work as intended. The problem is severe in recent as well as in older articles; for example, more than one-fourth of links published in the APSR in 2013 were broken by the end of 2014. We conclude that “reference rot” limits the transparency and reproducibility of political science research. We also describe practices that scholars can adopt to combat the problem: when possible, they should archive data in trustworthy repositories, use links that incorporate persistent digital identifiers, and create archival versions of the webpages to which they link.