This book develops and tests the “tolerance and tactics theory of negativity.” The theory argues that citizens differ in their tolerance of negative campaigning. Also, candidates vary in the tactics used to attack their opponents, with negative messages varying in their relevance to voters and in the civility of their tone. The interplay between citizens’ tolerance of negativity and candidates’ negative messages helps clarify when negative campaigning will influence citizens’ evaluations of candidates and their likelihood of voting. A diverse set of data sources was collected from U.S. Senate elections (e.g., survey data, experiments, content analysis, focus groups) across several years to test the theory. The tolerance and tactics theory of negativity receives strong empirical validation. First, people differ systematically in their tolerance for negativity, and their tolerance changes over the course of the campaign. Second, people’s levels of tolerance consistently and powerfully influence how they assess negative messages. Third, the relevance and civility of negative messages consistently influence citizens’ assessments of candidates competing for office. That is, negative messages focusing on relevant topics and utilizing an uncivil tone produce significant changes in people’s impressions of the candidates. Furthermore, people’s tolerance of negativity influences their susceptibility to negative campaigning. Specifically, relevant and uncivil messages are most influential for people who are least tolerant of negative campaigning. The relevance and civility of campaign messages also alter people’s likelihood of voting, and the impact of negative messages on turnout is more consequential for people with less tolerance of negativity.