ABSTRACT The article will evaluate the current state of UK-EU police and criminal justice cooperation and its implications for UK criminal justice policy. The article will place current developments within the broader historical and constitutional context of the ambivalent UK participation in the EU area of criminal justice before Brexit and highlight the significance of criminal justice and police practitioners in shaping UK European policy in the field. The article will map the constitutional evolution of the UK position in the field and assess the relevance of pre-Brexit challenges to the post-Brexit era. Looking at both how UK policy has been influenced – and has in turn influenced – EU criminal justice policy, the article will evaluate the provisions of the TCA and focus on key current challenges, including the constitutional constraints underpinning UK participation in EU police and criminal justice structures after Brexit, the challenge of regulatory divergence in the field (in particular regarding data protection) and the development of mutual trust and credible benchmarks of collaboration (including compliance with European human rights norms) after Brexit.