This study deals with the diversification of defence cooperation in the 21st century and proposes a typology for today’s cooperation. It classifies defence cooperation schemes according to i) their level of cooperation (bi-, mini- or multilateral), ii) their objectives (specific or general), iii) their length in time (transient or permanent), iv) their domain (related to operational aspects or investments), and v) their framework (institutionalised or not). Given the variety of cooperation forms, this study argues that it is necessary to examine their goals. More specifically, do states cooperate to ensure greater legitimacy or greater efficacy? Is it possible to combine the two? If so, what types and formats of cooperation should be favoured to achieve both efficacy and legitimacy? The study covers different defence cooperation schemes in the 21st century and questions their legitimacy and efficacy while taking into account the regional specificities of the cases being analysed.