The accidental discovery of the beneﬁts obtained for both fresh and hardened concretes when they are stabilized with 3.5% to 6% of air (35 to 60 litres per m3) in the form of a network of very tiny bubbles (10 to 100 micrometres in diameter) was a signiﬁcant step forward in the improvement of concrete durability. In this chapter, we will emphasize that some air should be entrained in all concretes, not only to improve their durability against freeze-thaw cycles, but also to improve their workability, a key factor when considering the durability and sustainability of concrete structures. We are convinced that entrained air is a key component of good concrete everywhere, not only in northern countries such as Canada. We are not alone in holding such a conviction. The Japanese also share this belief: systematically, they entrain 3.5% to 6% of air in all of their concrete in spite of the fact that resistance to freeze-thaw cycles is not a serious problem in Japan, except in the northern part of the Japanese archipelago, Hokaïdo Island.