Increased competition and the demand for efficiency in the nonprofit social service arena has created a movement towards innovative management practices in nonprofit organizations. In particular, the rise of interagency relationships among nonprofit social service organizations is a new phenomenon that promises to reshape these organizations in the coming millennium. This report couples a theoretical framework of interagency relationships among nonprofit social service organizations with the practical realities of creating such relationships. It begins by discussing the history of nonprofit organizations, focusing on social service agencies, and provides an overview of the challenges facing these organizations today. It then develops a framework for the terms used to describe inter-organizational relationships, ranging from informal collaboration to merger, as a means for addressing the management, resource, and service delivery challenges currently facing nonprofit social service organizations. From this perspective, five recent efforts by nonprofit social service agencies to form interagency relationships are highlighted. It concludes by identifying policy issues related to the trend towards interagency relationships and provides specific recommendations related to the development of such relationships to nonprofit leaders and funders.