Abstract European defence and security interests are at risk if undertakings and governments from other than EU countries can easily gain control of European defence and security equipment and technology. The authors therefore plead for the adoption of European rules designed to monitor foreign investments in the defence and security sectors. At this stage, such investments undergo no screening at all at EU level. National schemes exist in some Member States but can in an internal market easily be circumvented. Common rules designed to review the acquisition of substantial participations in European defence and security operators would considerably increase the autonomy, the security and the efficiency of the defence of the European Union and its Member States, particularly if implemented at EU level. Moreover they would form a corollary to the establishment of an internal defence market while at the same time supporting the reciprocity of the access of European industries to third country markets.