Recently I have been reading literature on informality as an element defining the shape and the social life of cities. This literature defends that in order to understand the development and growth of cities we have to look not only on the official policies promoted by local authorities but also to informal practices in housing, waste management, water and energy provision as well as in economic exchange. By informal practices we understand actions that non state actors develop to provide these services whithout the support of the state or even against the state. The impact of these practices in the development of cities is clear in expanding cities of the developing world, where new informal settlements emerge and become new neighbourhoods within the city with lack of basic services. The most known case are the Favelas of Rio de Janerio and other Brasilian cities, but we can find this kind of informal settlements in all cities of developing countries, such as Buenos Aires, Mumbai, or Johannesburg. Thus, urban slums are the result of the increasing process of urbanisation that humanity is facing.