This article analyses in a historical perspective the so-called acquis communautaire, i.e. the body of law, rules, including informal decisions/norms, taken by the European Community/European Union. In the first part of the article the early history of the concept from the establishment of the EC in 1958 to the early 1970s is analysed. The second part of the article analyses the impact of the acquis communautaire on the way the EC negotiated during the first successful enlargement negotiations from 1970 to 1972 with Denmark, Ireland, Norway and Britain. The article demonstrates empirically that even if national governments continued to play a leading role in European cooperation, the existence of a common body of law, rules and norms was difficult for single governments to set aside in their pursuance after of narrow national interest. This did not mean that laws could not be bent, or broken, but this required a broad alliance of member states, which was unopposed.