Faced with the growing world population, the fear of overcrowding has dominated for decades. Its veracity seems to be borne out by the notoriety of its proponents. However, any serious analysis shows that the population growth of the last two centuries is in no way explained by a rise in birth rates, but by the fantastic progress made by mankind in the fight against mortality. In the 21st century, this growth is set to slow down, which means that the 21st century will not be the century of so-called overpopulation, but rather that of a phenomenon unprecedented in its scale: the ageing of populations, albeit with considerable diversity from one region to another.