[en] Beyond broad policy declarations, the implementation of ecological transition - which consists mainly in curbing consumption of energy and raw materials in our societies - requires substantial behavioural change at the collective, but also, quite obviously, the individual level. Yet, though there is general consensus around the principle of embarking on the path to transition, things get more complicated when it comes to changing our practices and habits. Can we act on individual behaviour and collective dynamics in respect of this particular aim of ecological transition, and, if so, how are we to go about it? Solange Martin and Albane Gaspard have examined this question for the French Environment and Energy Management Agency (ADEME) and offer us the fruit of their labours here. They show, for example, how the social and human sciences help to understand behaviour both at the individual level and in its collective dimensions, and they outline different possible lines of action to modify it. But, given the entanglement between various levels, it is essential, if we are to act effectively on behaviour, to combine approaches, tools and actors, and to analyse and understand social practices thoroughly before implementing political projects or measures