The political parties in Congress are as polarized as they have been in 100 years. This book examines more than 30 years of congressional history to understand how it is that the Democrats and Republicans on Capitol Hill have become so divided. It finds that two steps were critical for this development. First, the respective parties’ constituencies became more politically and ideologically aligned. Second, members ceded more power to their party leaders, who implemented procedures more frequently and with greater consequence. In fact, almost the entire rise in party polarization can be accounted for in the increasing frequency of and polarization on procedures used during the legislative process.