Scholars have studied international NGOs as advocates and service providers, but have neglected the growing importance of these actors in enforcing international law. NGO enforcement comprises a spectrum of actions from indirect (e.g., monitoring and investigation), to direct (e.g., prosecution and interdiction). We explain the rise of these practices by the growing gap between the increasing legalization of international politics, and states’ limited capacity to enforce international laws, and by the diffusion of new technologies and legal changes facilitating non-state enforcement. There are important variations within this general trend: some NGOs engage in enforcement, while others stick with more traditional strategies. We explain this variance by organizational ecology dynamics. The proliferation of NGOs, and the resulting competition for scarce resources, encourages functional specialization within population niches, including enforcement, as actors seek to differentiate themselves from peers and competitors. We support these claims with evidence from the environmental and anti-corruption sectors.