This paper deals with the design of voting rules in the EU Council. Both internal and external impact of the voting rules are examined. Internal impact affects the distribution of power among the member states and external impact affects power relations between the main decision-making bodies in the EU. One of the main lessons of the analysis is that voting rules matter. This clearly explains why the design of Council voting rules has required so much bargaining and cumbersome marathon negotiations. The internal decisionmaking rules in the Council have substantial impact on both the national distribution of power in the Council and inter-institutional power between the EU's decision-making bodies.