Scholars do not usually test for the duration of the effects of mass communication, but when they do, they typically find rapid decay. Persuasive impact may end almost as soon as communication ends. Why so much decay? Does mass communication produce any long-term effects? How should this decay color our understanding of the effects of mass communication? We examine these questions with data from the effects of advertising in the 2000 presidential election and 2006 subnational elections, but argue that our model and results are broadly applicable within the field of political communication. We find that the bulk of the persuasive impact of advertising decays quickly, but that some effect in the presidential campaign endures for at least 6 weeks. These results, which are similar in rolling cross-section survey data and county-level data on actual presidential vote, appear to reflect a mix of memory-based processing (whose effects last only as long as short-term memory lasts) and online processing (whose effects are more durable). Finally, we find that immediate effects of advertising are larger in subnational than presidential elections, but decay more quickly and more completely. [Supplementary material is available for this article. Go to the publisher's online edition of Political Communication for the following free supplemental resource(s): discussion of methodological issues; results for a alternative specifications of key models; full reports of model results.]