This dissertation examines why intergovernmental organizations (IOs) adopt formal institutional mechanisms to structure relations with non-governmental organization (NGOs). Using insights from public administration theory, this dissertation argues that these formal engagement mechanisms help IOs obtain informational and/or reputational resources, and that IOs choose to adopt select mechanisms that maximize acquisition of these resources. To test my theory, I surveyed more than 120 regional and global IOs active in over fourteen policy areas. The statistical analysis of my survey data indicates that an IO selects the particular type of engagement mechanism that best meets its informational and/or reputational resource needs. To contextualize these findings, I conducted in-depth historical case studies of four IOs. These case studies suggest that IOs adopt multiple mechanisms in response to changes in both their internal organizational environment and their external political environment. My work contributes to the literature on global governance by demonstrating how the resources provided by NGOs shape institutional structures at international organizations, and how these structures reflect IO perceptions about their relations with non-state actors.