Weblogs occupy an increasingly important place in American politics. Their influence presents a puzzle: given the disparity in resources and organization vis-à-vis other actors, how can a collection of decentralized, nonprofit, contrarian, and discordant websites exercise any influence over political and policy outputs? This paper answers that question by focusing on two important aspects of the “blogosphere”: the distribution of readers across the array of blogs, and the interactions between significant blogs and traditional media outlets. Under specific circumstances – when key weblogs focus on a new or neglected issue – blogs can socially construct an agenda or interpretive frame that acts as a focal point for mainstream media, shaping and constraining the larger political debate. These arguments receive support from a network analysis of blog links, as well as a survey of media professionals about their blog preferences.