In this paper we examine the roles and impact of science advice in an arena of public decision-making which, although critical to the healthy functioning of democratic societies has largely been neglected in recent discussions: that of elected legislatures. We begin by contrasting the functions and operating environments of legislatures with those of executive branches of governments, and consider how their differences affect the requirements of their respective science advisory systems. Several examples of science advice organisations for national legislatures in Europe are then examined, revealing a range of advisory structures and practices. These variations in approach suggest different underpinning models about both why science advice is being provided, and how science advice influences a legislatures’ activities. Given these differences, and our currently limited understanding of the influence of research knowledge in any sphere of public decision-making, we argue that more attention needs to be focused on the difficult challenge of mapping and evaluating the impact of science advice on legislatures’ democratic functions. This argument is illustrated using a recent case of science advice directly influencing democratic decision-making within the UK legislature on the treatment of mitochondrial disease. Finally, we conclude the paper by describing a study on the impact of science advice currently underway in the UK legislature, and propose a multi-legislature comparative expansion of this study to improve significantly our understanding of the influence science advice has on the functions of democratic societies’ deliberative bodies.