This report focuses on the Government's approach to the opt-in Protocol, introduced by the Lisbon Treaty, by virtue of which the UK has a right not to participate in EU justice and home affairs (JHA) measures. At issue is whether the opt-in Protocol can be interpreted to mean that it is the content of an EU measure which determines the application of the Protocol, rather than a legal base under the JHA title of the Treaty on the Functioning of the EU (Title V). The Committee express no view on the desirability or otherwise of the opt-in mechanisms introduced by the Lisbon Treaty. The function of this report is to examine the way in which the Government has sought to interpret those mechanisms.