The phenomenon of the rebound effect has been known for decades now, yet it is very much absent from resource efficiency policies. One of the reasons is that there is a plethora of different estimates for the rebound effect, depending not only on the country and the sector studied but also on the level and type of rebound effect addressed. This chapter aims, in a first step, at enhancing the theoretical foundations of the rebound effect. Based on previous work available in the literature, it develops a comprehensive classification that distinguishes the different levels of the rebound effect and the mechanisms at play. In a second step, this chapter discusses what can be learned from the mere concept of the rebound and its implications for the sustainability debate. Two statements are addressed: (1) the rebound effect challenges the feasibility of an absolute decoupling between economic growth and resource use, and (2) improved resource efficiency alone cannot counterbalance the high environmental impact of P (population) x A (affluence). Given that, it is argued that the existence of the rebound effect implies that resource efficiency policies are not enough to achieve sustainability and can even be detrimental in the case of backfire. It is thus time to shed light on the pathways “producing and consuming differently” and “producing and consuming less”.