They believe EU cooperation makes it easier to keep criminals and terrorists out of the UK. Since 2004, using the European Arrest Warrant, more than 1,000 suspects have faced justice in UK courts and more than 7,000 have been extradited.
They argue that the extradition of smugglers and those involved in organised crime from Northern Ireland to Ireland and vice-versa would be made more difficult.
They believe leaving the UK would undermine cooperative efforts with EU countries to limit people trafficking and human slavery.
They argue that the European Court of Justice makes it more difficult to deport violent criminals.
UKIP leader, Nigel Farage, believes leaving the EU would prevent innocent British people from being put in prison under the European Arrest Warrant system.
The Leave camp believes the UK will be able to develop a new extradition treaty with the EU to ensure that the UK continues to easily arrest and extradite serious criminals such as terrorists and paedophiles.