This article explores the newly founded European Research Council's (ERC) peer review system and its ability to sustain its mission to promote excellent, groundbreaking research. The article explores the extent to which the selection of groundbreaking research is constrained by inherent limitations in peer review by analysing the informal practices of ERC peer reviewers. This article notes that controversy and uncertainty are central characteristics of potentially groundbreaking research proposals. The selection of truly innovative research is constrained by the boundaries on current knowledge, against which the value of proposed research is judged; these boundaries affect the extent to which peer review panellists feel they can take risks in their judgments and the rules of interpretation and deliberation they adopt. The role of customary interpretative rules is to limit the risks involved in decision making. Predicting the outcomes of peer review in controversial situations is difficult, however, as contingent factors play an important role. Copyright The Author 2012. Published by Oxford University Press. All rights reserved. For Permissions, please email: firstname.lastname@example.org, Oxford University Press.