The mix is proper to a mixture of heterogeneous elements or different character. Social diversity as the co or cohabitation in the same place people or groups socially, culturally or even of different nationalities. This leads us to consider the social mix as the co or cohabitation in the same place people or groups socially, culturally or even of different nationalities. The mix would be a cure for the ills of modern city, see generated by it. Taking the usual comments, whether from the press or urban studies and without prejudging their reality or can mention the contrast between rich areas and poor areas, difficult areas that go adrift, "ghettos" problems also raised by what would be a crisis of social bond, "divide", which would result in the increase in crime, the development of anonymity, individualism, exclusion. The urban sprawl, museification center are also discussed. Mainly mixed should ensure mixing of different social categories, brewing itself being a guarantor of social cohesion. In this way, the mix would be the guarantee of harmony in difference. So gather the diversity of social groups in the city is a concern that does not start today or even yesterday. This dream has lived the great utopian thought from Plato to Thomas More. The pioneers of modern urbanism designed projects for some forms of social diversity. That said, the theme of social diversity is mobilized as a central objective of urban policy. These policies are designed to cope with the rise of urban poverty analyzed in terms of social and urban exclusion. They are based on the model of a "balanced" society seamlessly blending social classes, ethnic groups and generations to ward crystallization pockets of poverty that would be problematic because at the basis of a culture of poverty, development insecurity or the establishment of an "underclass".