From Trump’s infrastructure plan in the United States and the German coalition agreement to the Spinetta report on rail transport or the recent revision of the Grand Paris Express schedule in France: now is the time for infrastructure investments. Even formerly reluctant states — one need only think of Germany — have converted to the urgent need to invest in infrastructure. Why now? The crystallization of multiple factors explains this enthusiasm: the conclusion of the 2008 crisis, which came to an end in 2015 in Europe; the availability of plethoric liquidities; the increasingly manifest decay of an infrastructure park inherited from 30 years of post-war economic growth; the ecological transition, already well underway as far as electricity is concerned, but still in its infancy in the field of transport; and, as always, the affirmation of the metropolitan fact, resulting in the continuous growth of large cities. “Infrastructure is back”: Act One.