Cities, whether those that grow “naturally”, or those that are planned, such as Brasília, Goiânia or Belo Horizonte, wind up with an organic life as if – or because they are – living beings. When we think of sustainability we must use not only the methods of urban ecology but the paradigms of biology as well. A city can grow considerably without “getting fat” and without creating new streets or neighborhoods. It can grow in a sustainable way by occupying empty spaces. Thus, the first mission to have sustainable cities is to occupy empty spaces in a coordinated and intelligent manner. The second important mission is to make the city grow at the costs of those who will benefit from this growth - organic and rational growth. The third large mission to achieve sustainable cities is to apply Urban Planning and Architecture throughout the urban fabric and the entire built environment. We must share with all layers of the population, rich and poor, workers and unemployed, not only a roof, but also the right to Architecture, Urban Studies and quality built space.