What impact will the Lisbon Treaty's new system of voting in the Council have? After describing the new voting rules, this study develops a modelling approach to assess their likely impact. The first part of the analysis examines the extent to which procedural rules have affected the decision-making process in the recent past, since this will help assess the likely impact of the changes brought by the Lisbon Treaty. The second part of the analysis presents a counter-factual analysis of recent decisions, exploring what would have happened had the Lisbon rules been applied. The main finding is that even under the strong and unrealistic assumption that formal rules define the decision-making process, decision outcomes would have been the same in most cases. The paper concludes by discussing the features of Council decision-making that soften the impact of these rule changes.