Widespread government contracting for nonprofit social service delivery has resulted in extensive reliance on networks of service providers, which involve complicated accountability dynamics. The literature has tended to emphasize formal aspects of accountability in contract relationships, focusing on the specification of contract terms, performance measures, reporting relationships, and stipulated consequences. Far less attention has been focused on the interorganizational and interpersonal behaviors that reflect informal accountability. This article examines the informal norms, expectations, and behaviors that facilitate collective action and promote informal accountability among nonprofit network actors. The data are based on in-depth interviews with nonprofit senior administrators in four major metropolitan areas. Based on this research, the authors propose a preliminary theory of informal accountability that links (1) the shared norms and facilitative behaviors that foster informal accountability for collective outcomes, (2) the informal system of rewards and sanctions used to promote and reinforce behavioral expectations, and (3) the challenges that may undermine informal accountability.