Our main concern is to understand Senate party development. What are the problems that individual legislators encounter in the absence of leadership? How do they set out to solve problems of coordination and collective action? Our answer, and our central argument, focuses on three factors: party competition, factionalism, and entrepreneurs. In the Senate, where leadership and institutional organization rest in the two parties rather than in the presiding officer, members adopt innovative structures when parties are most closely balanced. With this book, we look at the rise of party organization and leadership in the Senate throughout its history—showing the origins of the Senate caucus in the 1840s, the Republican steering committee in the 1880s and early 1890s, and Senate floor leadership in 1890, and then analyzing the maturation and development of party leadership and organization in the twentiethth and twenty-first centuries. We focus on five main features of Senate leadership: party organization management, floor management, service as intermediary with the president, party spokespersonship, and coalition building.