This paper examines how the Internet is changing the ideological segregation of the American electorate. Using individual and aggregate data, the authors analyze online news consumption, offline news consumption, and face-to-face social interactions to define ideological segregation using standard indices from the literature on racial segregation. They find that ideological segregation of online news consumption is low in absolute terms, higher than the segregation of most offline news consumption, and significantly lower than the segregation of face-to-face interactions with neighbors, co-workers, or family members. They find no evidence that the Internet is becoming more segregated over time. The authors measure ideological segregation using the "isolation index," which is the average conservative exposure of conservatives minus the average conservative exposure of liberals. They find that the isolation index for the Internet is 7.5 percentage points, the difference between the average conservative's exposure and the average liberal's exposure. They compare this to other media, finding that the Internet is less segregated than broadcast television, magazines, cable television, and local newspapers, but more segregated than national newspapers. The authors also find that online segregation is somewhat higher than that of a social network where individuals are matched randomly within counties, and lower than that of a network where individuals are matched randomly within zipcodes. They find that the Internet is significantly less segregated than actual networks formed through voluntary associations, work, neighborhoods, or family. The Internet is also far less segregated than networks of trusted friends. The authors explore two economic mechanisms that limit the extent of online segregation. First, online news is vertically differentiated, with most consumption concentrated in a small number of relatively centrist sites. Second, a significant share of consumers get news from multiple outlets. This is especially true for visitors to small sites such as blogs and aggregators. Visitors of extreme conservative sites such as rushlimbaugh.com and glennbeck.com are more likely than a typical online news reader to have visited nytimes.com. Visitors of extreme liberal sites such as thinkprogress.org and moveon.org are more likely than a typical online news reader to have visited foxnews.com. The authors also find that ideological segregation at the level of individual stories may differ from segregation at the level of the news outlet. They present evidence from case studies of two major news events—the Virginia Tech shootings in 2007 and the presidential election in 2008. On both of these days, the number of hits to news websites spikes significantly, and most content consumed presumably focuses on these major events. The isolation index for these days, however, is if anything lower than on an average day. These cases provide some evidence that online segregation is low even when within-outlet sorting is limited, and that conservatives and liberals are not highly segregated in their sources for information about major news events. The authors conclude with an important caveat: none of the evidence here speaks to the way people translate the content they encounter into beliefs. People with different ideologies see similar content, but both Bayesian and non-Bayesian mechanisms mayThis paper examines how the Internet is changing the ideological segregation of the American electorate. Using individual and aggregate data, the authors analyze online news consumption, offline news consumption, and face-to-face social interactions to define ideological segregation using standard indices from the literature on racial segregation. They find that ideological segregation of online news consumption is low in absolute terms, higher than the segregation of most offline news consumption, and significantly lower than the segregation of face-to-face interactions with neighbors, co-workers, or family members. They find no evidence that the Internet is becoming more segregated over time. The authors measure ideological segregation using the "isolation index," which is the average conservative exposure of conservatives minus the average conservative exposure of liberals. They find that the isolation index for the Internet is 7.5 percentage points, the difference between the average conservative's exposure and the average liberal's exposure. They compare this to other media, finding that the Internet is less segregated than broadcast television, magazines, cable television, and local newspapers, but more segregated than national newspapers. The authors also find that online segregation is somewhat higher than that of a social network where individuals are matched randomly within counties, and lower than that of a network where individuals are matched randomly within zipcodes. They find that the Internet is significantly less segregated than actual networks formed through voluntary associations, work, neighborhoods, or family. The Internet is also far less segregated than networks of trusted friends. The authors explore two economic mechanisms that limit the extent of online segregation. First, online news is vertically differentiated, with most consumption concentrated in a small number of relatively centrist sites. Second, a significant share of consumers get news from multiple outlets. This is especially true for visitors to small sites such as blogs and aggregators. Visitors of extreme conservative sites such as rushlimbaugh.com and glennbeck.com are more likely than a typical online news reader to have visited nytimes.com. Visitors of extreme liberal sites such as thinkprogress.org and moveon.org are more likely than a typical online news reader to have visited foxnews.com. The authors also find that ideological segregation at the level of individual stories may differ from segregation at the level of the news outlet. They present evidence from case studies of two major news events—the Virginia Tech shootings in 2007 and the presidential election in 2008. On both of these days, the number of hits to news websites spikes significantly, and most content consumed presumably focuses on these major events. The isolation index for these days, however, is if anything lower than on an average day. These cases provide some evidence that online segregation is low even when within-outlet sorting is limited, and that conservatives and liberals are not highly segregated in their sources for information about major news events. The authors conclude with an important caveat: none of the evidence here speaks to the way people translate the content they encounter into beliefs. People with different ideologies see similar content, but both Bayesian and non-Bayesian mechanisms may