The civil society sector-made up of millions of nonprofit organizations, associations, charitable institutions, and the volunteers and resources they mobilize-has long been the invisible subcontinent on the landscape of contemporary society. For the past twenty years, however, scholars under the umbrella of the Johns Hopkins Comparative Nonprofit Sector Project have worked with statisticians to assemble the first comprehensive, empirical picture of the size, structure, financing, and role of this increasingly important part of modern life. What accounts for the enormous cross-national variations in the size and contours of the civil society sector around the world? Drawing on the project’s data, Lester M. Salamon, S. Wojciech Sokolowski, Megan A. Haddock, and their colleagues raise serious questions about the ability of the field’s currently dominant preference and sentiment theories to account for these variations in civil society development. Instead, using statistical and comparative historical materials, the authors posit a novel social origins theory that roots the variations in civil society strength and composition in the relative power of different social groupings and institutions during the transition to modernity. Drawing on the work of Barrington Moore, Dietrich Rueschemeyer, and others, Explaining Civil Society Development provides insight into the nonprofit sector’s ability to thrive and perform its distinctive roles. Combining solid data and analytical clarity, this pioneering volume offers a critically needed lens for viewing the evolution of civil society and the nonprofit sector throughout the world.