In this paper we examine the role of nongovernmental organisations (NGOs) in debates about environmental science and knowledge, using empirical evidence from in-depth interviews with a range of NGOs involved in the waste debate in the United Kingdom. We discuss theoretical issues of scientific boundary-work and the construction of expertise and socially distributed knowledge, and then apply these to our empirical evidence. Our conclusions are that NGOs continue to subscribe to the notion of the preeminent authority of science in environmental debates, but also work partly in a more diverse, highly networked world of knowledge production which requires them to be pragmatic and versatile in how they legitimate knowledge from various sources. Hence, scientific knowledge is highly contingent in its authority, and dependent upon continual (re)negotiation.