A party machine is treated here as a social institution whose development is determined primarily by rapid urbanization and industrialization after the Civil War. It is a social institution whose time has come. The relations between the party organization via local bosses with the mass of its poor clients illustrates how the mechanism of social exchange become a formula for machine’s power resting on techniques of control, corruption, and electoral entrepreneurship. It is interesting to observe how the patron – client setting works both for the new immigrants as mass of loyal voters and business elite as material beneficiaries of corruption. For the immigrants involvement in the machine politics is an important element of Americanization and civic education. After analyzing the intricacies of machine politics the focus of attention switches to the middle class reformers who, incited by investigative journalists (the muckrakers), embark on campaigns to replace politics of interest and personal ambition in city affairs with rational management of assets in the hands of professional experts. The clash between bosses and reformers illustrates the ever going confrontation between two visions of democracy and gives this study of machine politics in American cities during the Progressive Era a relevance extending beyond one historical period.