The Maastricht Treaty obliges EU members to adopt the euro upon meeting certain monetary and budgetary convergence criteria, but not all the member states have to do so. Denmark and United Kingdom negotiated exemptions, so they keep to use their own official currencies.
EU members can adopt Euro when they meet the convergence criteria, also known as the Maastricht criteria. It usually takes longer period of time to meet them, so the adpotion of Euro may be not so simple, because it requires also economic stability. Sweden turned down the euro in a non-binding referendum in 2003, but they will probably adopt it in the future. Czech republic and Croatia are expected to adopt euro soon, in a couple of years.