This 1970 collection of specially commissioned papers develops the theory of both long- and short-distance migration. The movement of human population, which has always been an essential ingredient of social change, tended in the past to be considered primarily in terms of economic interest. It embraces, however, many complex aspects of human aspirations and changing circumstances that are developed in the initial decision to migrate taken in the community of origin and the process of adjustment and settlement in the community to which the migrant comes. The importance of the factors related to immigration, especially of those from widely differing national and racial groups, is very apparent in Britain and many other countries. These more widely dramatized migrations are part of a social process that has common features with the movement from country to town in the same country, and is closely related to changes in the family cycle, to social mobility and economic development.