This paper examines the possible impact of Brexit on EU-UK naval and maritime defence co-operation, especially with regard to issues of procurement and strategy. It is structured around two scenarios: “Rupture: Naval co-operation after a no deal Brexit” and “Continuity: naval co-operation under a Brexit grand bargain.” From the perspective of the Royal Navy, the formula of ‘leaving the EU but not Europe’ is ambiguous. On the one hand what appears to be sacrificed is future British participation in EU naval missions such as Operation Atalanta, and with that influential senior command positions, together with the migration of command and control functions from Northwood to Rota in Spain. On the other hand, it is entirely plausible the Royal Navy will continue to be a vital contributor to European led maritime security missions, either in an ad hoc manner or under a parallel NATO command/deployment. After all, if Norway or Denmark have proven their ability to work alongside EU naval forces why shouldn’t the British? One spoiler in this otherwise comforting ‘continuity’ scenario may well be British strategic culture and the long-term drift in British elite conceptions of national interests. For if British defence elites decide to privilege naval deployments, operations and presence in the Middle East or Pacific, further reinforcing a special relationship with the US, this may lead to hard choices, given shrinking navies and numerous threats. Whereas the British Army will likely continue to find an important role for itself in defending the Baltics, or NATO’s other flanks, for the Royal Navy and British maritime industries, the Brexit discourse of ‘Global Britain’ has a strong appeal. Conversely, the EU has stepped up its ambition on defence issues, with initiatives such as PESCO and a European Defence Fund. For now, these are of such small significance as regards naval co-operation as to be no great loss from a British perspective. However, likely to be much more important is a wave of strategic merger activity, between French, Italian and German shipyards. If these trends solidify towards greater collaborative European naval procurement and operations, and possibly increased European ‘strategic autonomy’, they should lead to greater divergences between European and British naval forces, at least as regards the types of ships, weapons and capabilities acquired. However, given extensive overlapping and evolving NATO membership across the EU, it seems unwise to assume the Royal Navy will retreat from European waters or that the UK’s naval clout won’t still count in European security.