First Joint Report from the European Scrutiny, Home Affairs and Justice Committee - 19th March 2014).
The UK government had the right under the Lisbon Treaty to opt-out of some 135 EU measures relating to justice and home affairs - (post of 19th October 2012). They also had the right to select which ones to opt back into and it seems that the government has chosen to opt back into around 35 of those measures including the European Arrest Warrant which came to the forefront of the news this week in the Ashya King case.
The Guardian - Ashya King case gives Eurosceptics a chance to air Lisbon grievances - has suggested that "the government is now keen to get the last crucial bit of the deal done
with a minimum of fuss, but Tory MPs have successfully extracted a
commitment that the whole matter will be debated again and subjected to a
Despite the undoubted importance of this, it cannot be said that the politicians have made it easy for the public to see precisely what is being opted into and, for that matter, what has been opted out of.
The most straightforward publication I have found so far is this Commons Standard Library Note dated 15th July 2013 which considers, as things stood at that date, the bloc opt-out and selective opt-back-ins.
BBC News 10th July 2014 - House of Commons debate
UK government - JHA opt-in and Schengen opt-out proposals (Updated 16th June 2014)
Parliament - EU justice and home affairs opt-out
Cm 8671 - July 2013 - Decision pursuant to Article 10 of Protocol 36 to the Treaty on the Functioning of the EU - a list of the opt-ins appears at pages 8 to 12.
Cambridge University Faculty of Law - Centre for European Legal Studies September 2012 - Opting out of EU Criminal Law: What is actually involved?
Professor Steve Peers - Professor of Law University of Essex - Statewatch - The UK Opt out from Justice and Home Affairs Law: the other member States finally lose patience