In general, the law of the European Union is valid in all of the twenty-seven European Union member states. However, occasionally member states negotiate certain opt-outs from legislation or treaties of the European Union, meaning they do not have to participate in certain policy areas. Currently, three states have such opt-outs: Denmark (two opt-outs), Ireland (two opt-outs) and Poland (one opt-out). The United Kingdom had four opt-outs before leaving the Union.
This is distinct from the enhanced cooperation, a measure introduced in the Treaty of Amsterdam, whereby a minimum of nine member states are allowed to co-operate within the structure of the European Union without involving other member states, after the European Commission and a qualified majority have approved the measure. It is further distinct from the Mechanism for Cooperation and Verification, whose lifting is conditional on the relevant member states meeting certain benchmarks, and temporary derogations from certain areas of cooperation (such as the Schengen Agreement and the eurozone) until the relevant member states satisfy the entry conditions.