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. The Committee examines the Government’s interpretation of the expression pursuant to [Title V] in the opt-in Protocol, and conclude that it has an accepted legal meaning, namely that a Title V legal base is required before the opt-in can be triggered. As a consequence, the Committee recommends that the Government reconsider its broader interpretation.