One of our great urbanists and one of our great public health experts join forces to reckon with how cities are changing in the face of existential threats the pandemic has only accelerated Cities can make us sick. That's always been true-diseases spread more easily when more people are close to one another. And cities have been demonized as breeding grounds for vice and crime from Sodom and Gomorrah on. But cities have flourished nonetheless because they are humanity's greatest invention, indispensable engines for creativity, innovation, wealth, and civilization itself. But cities now stand at a crossroads. During the global COVID crisis, cities grew silent; the normal forms of socializing ground to a halt. How permanent are these changes? Advances in technology mean that many people can opt out of city life as never before. Will they? Are we on the brink of a post-urban world?