Current literature on global environmental politics largely considers NGO influence implicit and unproblematic. Responding to several weaknesses in the literature, we propose a coherent research framework for assessing NGO influence in international environmental negotiations. We contend that influence can be said to have occurred when NGOs intentionally transmit information that alters the behavior of negotiators, and call for collecting and analyzing evidence of NGO influence in a more systematic fashion. Our framework, which relies on the use of multiple data types, sources, and methods, controls for over-determination and allows researchers to identify, with a sound degree of confidence, instances of NGO influence in international environmental negotiations. The resulting comparability provides a basis for analysis of NGO influence across cases, and ultimately contributes to better understanding of the variation of NGO influence in global environmental politics.