During his academic career Milton Santos published approximately 40 scientific works on the so-called Third World. This period of intellectual activity corresponds to the time the geographer was forced to live in an exile that started in France. He mainly wrote about the theme from the point of view of urban agglomerations. To present the reality of these Third World cities, he frequently used figures and empirical studies. He also observed that the dynamism and the expansion of cities in underdeveloped countries depended more on their population growth than on their economy. Milton Santos thought these major cities in the Third World could not be studied as whole systems and his observations led to the elaboration of the theory of the two circuits. The notion, first introduced in the last chapter of Santos's book Les Villes du Tiers-Monde, was further developed in one of his most accomplished works on the theme: L'Espace Partage: Les Deux Circuits de l'Economie Urbaine des Pays Sous-Developpes. The author identifies the upper circuit as the direct result of a modernization process that drastically changes modes of production and communication, as well as lifestyle and consumption patterns. He understands the lower circuit as a consequence of the same modernization processes, but in a more indirect manner: a circuit that includes people and activities that do not benefit at all, or benefit only partially, from recent technological advancements. However, Santos considers these two circuits closely dependent and operating with one another.