Efficiency is still a pervasive idea in economic literature. This paper aims to discuss the relevance of this notion. It argues that efficiency is useless in neoclassical economics - where efficiency is a necessary consequence of the postulate of full rationality - and is meaningless in a complex system view - where the means-ends framework does not apply globally. Implications for environmental economics and for ecological economics are drawn. Finally, it is pointed out that, even without efficiency, collective action can still play a major role.