Although firms in diverse industries increasingly adopt private regulation of labor standards for workers in their global supply chains, growing scholarly evidence suggests that this approach has not generated sustainable improvements in working conditions for those workers. The authors draw on recent developments in institutional theory regarding the development of opaque institutional fields that cause the decoupling between practices and outcomes to develop a new explanation for the lack of sustainable improvement in labor practices in supply chains. Using qualitative and quantitative data from a global apparel supplier and a global home products retailer, they demonstrate the various ways in which opacity causes decoupling between private regulation practices of global firms and outcomes for workers in supply chains.