Denmark has four opt-outs from EU co-operation. The Danish opt-outs stem from 1993 and were agreed amongst the then 12 Member States after the Danish population initially rejected the Maastricht Treaty in a referendum in 1992. The opt-outs are outlined in the so-called Edinburgh Agreement and concern the Monetary Union (EMU), Common Security and Defence Policy (CSDP), Justice and Home Affairs (JHA) and the Citizenship of the European Union.
Denmark has held two referenda on the opt-outs. In year 2000 the Danes voted "no" to the Euro. As a consequence Denmark is not part of the Euro group and has kept its currency the kroner. In December 2015 the Danes voted "no" to replacing the Danish opt-out with an opt-in model for Denmark's participation in Justice & Home Affairs.