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Since the pioneering work of Williamson, several authors have shown that urban concentration in a country tends to increase during economic take-off and industrialization and to decrease beyond a given per capita income of about $5,000 (in 1985 dollars). Based on new findings from economic geography, this paper provides new insights into the fact that in developing countries, the unbalanced spatial distribution of activities and long-term urbanization are largely determined by the development stage and the associated evolution of productive specialization that characterizes a country.