Given the fact that the Lisbon Treaty is rather similar to the Constitutional Treaty in content, it looks surprising that Denmark could avoid a referendum on the former when it had previously been decided that a referendum would have been necessary to ratify the latter. This paper will give some background on the Danish Constitution and transfer of sovereignty to supranational organisations and give a brief overview of how these rules have been applied in connection with the Accession Treaty in 1972 and subsequent treaty reforms. Denmark ratified the Treaty of Nice without a referendum. The decision whether to have a referendum is officially a legal one, but one can ask whether political considerations enter too. Given the negative referendum votes on the Maastricht Treaty in 1992 and on joining the euro in 2000 the government is eager to avoid the use of referendums. The reason for this is partly that the Danish public is sceptical about European integration. This affects parliamentary politics and it makes treaty reform a difficult two-level game in Denmark.