Despite a recent emphasis on 'evidence based policy' accompanied by an abundance of 'green' policy instruments, experience from the European Union and OECD countries shows that decisions which truly aim to balance environmental considerations with social and economic ones remain thin on the ground. Moreover, many policies seem to fall short of, or directly contradict what the available 'evidence' suggests is required. This is a synthesis paper bringing together literature from the fields of political science, geography, sociology and science and technology studies to outline some of the obscurities relating to the use of scientific evidence in environmental decision-making. In this paper, we suggest that an exploration of three key inter-related issues is necessary to develop a richer understanding of why evidence and policy interact as they do. These are the nature of evidence itself; the normative, moral or ethical 'politics' of policy-making; and the operation of power in the policy process. Our primary goal is to bring various literatures together to better conceptualise the evidence-policy relationship. In so doing, we outline specific challenges for knowledge producers who set research priorities, and design and direct research projects. We also highlight significant implications for policy decision-making processes.