The relationship between evidence, governance and institutions in the pursuit of sustainable development is notoriously complicated. Studying organisations whose roles include managing boundaries between evidence and policy is one way to better understand this relationship. But in spite of the complexity, such organisations often appear – officially at least – to have rather limited remits, with very sharply drawn boundaries. This paper investigates this puzzle through study of the United Kingdom Parliamentary Environmental Audit Committee (EAC), established in 1997 to scrutinise government departments’ efforts to integrate environment and sustainable development into policymaking. Gathering and deployment of evidence are critical to its work, and it is able to call as witnesses both members of the executive and experts from outside government. Drawing on Jasanoff’s work on issue framing and boundaries, and Owens and Rayner’s work on Royal Commissions, this paper employs a broad definition of boundary work to allow potential identification of multiple boundaries shaped by different sorts of boundary work. It investigates what boundaries are drawn, how, why and by whom. Through analysis of EAC reports, and elite interviews, the paper examines the context, consultation processes, roles played and influence of the EAC. The committee is found to play many roles, including analyst, forum for debate and political lever, all of which provide potential for influence on specific policies, and on the nature and space of political debate. The EAC shapes many boundaries using various mechanisms, both informal and institutionally-sanctioned. The rich and subtle variety of these boundaries, and different work carried out around them, confirms that a simple reading of the EAC’s remit of government scrutiny falls short of understanding how it works in practice. The results vividly illustrate the politicised nature of sustainable development policy, and informs prospects for ‘filling’ any gap between evidence and policy.