Quality is all the more difficult to define as its meaning seems obvious. It’s worth considering the different senses of the word and trying to explain how quality can, at the same time, be forgotten by sciences, while being so much used in the field of management. Hence, a more precise meaning can be given. This allows explaining why official approaches of sustainability have reached deadlock. In fact, quality appears to be a precious operator, that reveals which characteristics of beings or things are really important for actors.