This two-part special issue of American Behavioral Scientist (that is, this volume and the one that will follow it in July 2002) emerged from the work of the Social Science Research Council's Committee on Research on Philanthropy and the Nonprofit Sector. The purpose of the committee is to help research on philanthropy and the nonprofit sector to achieve greater visibility, greater coherence, intellectual momentum, and intellectual direction. A central part of this fieldbuilding initiative is to promote the development of intellectual and material resources for studying philanthropy and the nonprofit sector. Toward this end, the committee decided early in its work to examine the quality and availability of data on nonprofit organizations and philanthropy by commissioning the essays included in this volume.