The lack of predictive power over complex systems either designed by humans or evolved by nature, is a foundational problem in contemporary science. Real networks such as the Internet, the Web, social and biological networks, have acquired emergent large-scale properties that are beyond our full understanding, much less prediction or control. Astonishingly, these emergent properties are the same across networks from drastically different domains, and are the ones required to facilitate the optimality of some important network functions, such as information transport.