ABSTRACT : Definitions of cooperative action usually provide sufficient rather than necessary conditions. I here define a minimal form of cooperation, which encompasses mass actions, such as demonstrations. Intentional, epistemic, strategic and teleological aspects, mainly obtained from a weakening of pre-existing concepts, are discussed. I also emphasize the key role played by game theoretic concepts in the definition of joint action. Finally, I claim that rationality is a necessary condition of minimal cooperation, whereas classical analysis merely sees rationality as a possible or desirable characteristic.