The Treaty of Lisbon contains several options for creating some degree of flexibility in the defence domain under the assumption that it will be difficult, and not always necessary, to do everything at 27: constructive abstention, the possibility of mandating a smaller group or for the most demanding missions ‘permanent structured cooperation in defence’ (PSCD). It is not yet clear, however, how the possession of larger military capabilities and how the will to enter into more binding commitments will be elaborated. This paper traces the debate on flexibility in the area of security and defence and addresses, in particular, PSCD and its potential application.