The concepts of ethnie and nation have become so loaded with values and passions that it is necessary to define them in order to be able to employ them in sociological research. Ethnie designates an historical community, which is conscious of being unique and display the desire to remain so. But, contrary to nation, it does not necessarily have a political organization. A double reflection on the modern political nation is elaborated. The first is based on the legitimacy of the will of the people (French — or American — style nation). The other is based on ethnic and linguistic criteria (German style nation). It is important to go beyond these historical and ideological oppositions and propose a sociological definition of the modern nation, based on the process of political integration of populations, diversified in terms of their origins and their characteristics, around a shared political project.