Scholars have studied international NGOs as advocates and service providers, but have neglected their importance in autonomously enforcing international law. We have two basic aims: first to establish the nature and significance of transnational NGO enforcement, and second to explore the factors behind its rise. NGO enforcement comprises a spectrum of practices, from indirect (e.g., monitoring and investigation), to direct enforcement (e.g., prosecution and interdiction). We explain NGO enforcement by an increased demand for the enforcement of international law, and factors that have lowered the cost of supply for non-state enforcement. Increased demand for enforcement reflects the growing gap between the increased legalization of international politics and states’ limited enforcement capacity. On the supply side, the diffusion of new technologies and greater access to new legal remedies facilitate increased non-state enforcement. We evidence these claims via case studies from the environmental and anti-corruption sectors.