The Schengen Area currently consists of twenty-six states, all but four of which are members of the European Union(EU). Two of the non-EU members, Iceland and Norway, are part of the Nordic Passport Union and are officially classified as states associated with the Schengen activities of the EU. The third, Switzerland, was subsequently allowed to participate in the same manner in 2008. The fourth, Liechtenstein, joined on 19 December 2011, becoming the newest member of the Schengen Area. De facto, the Schengen Area also includes several microstates that maintain open or semi-open borders with Schengen countries. Two EU members—Ireland and the United Kingdom—negotiated opt-outs from Schengen and continue to operate systematic border controls with other EU member states.
Before fully implementing the Schengen rules, each state needs to have its preparedness assessed in four areas: air borders, visas, police cooperation, and personal data protection. This evaluation process involves a questionnaire and visits by EU experts to selected institutions and workplaces in the country under assessment
