Following the EU exit referendum on 23rd June 2016, the exact conditions of the UK’s withdrawal from the EU were negotiated against a tense backdrop of domestic instability, characterised by political division and public protest. During this period, the Brexit debate understandably dominated the media, with commentators frequently referring to the risks associated with any loss of access to the Schengen Information System (SISII) by UK law enforcement agencies. This chapter will examine the operational functionality of SISII, the policing implications of its loss for UK law enforcement agencies, and the post-Brexit steps taken to mitigate the loss of operational capability.