INTRODUCTION The three main sources of urban population g gration, natural increase and the adminis- reclassification of urban areas, with the first trative of greatest importance. In general, the two being contribution of migration and natural elative increase to urban growth varies with level of urban- isation (see Chapter 4). At an early stage of devel- opment, when levels of urbanisation are low and rates of both urban and rural natural increase are moderately high, net migration generally contrib- utes more to urban population growth than natural increase. At an intermediate stage of urbanisation, natural increase predominates. At a later stage, with a high level of urbanisation and low ratc of natural increase, the balance reverts in favour of net migra- tion. Although some Third World countries are now in the intermediate stage and many large cities are growing more from natural increase, migration remains a major factor in urban growth and, in view of the size of the rural reservoir of potential migrants, will continue to be so for the foreseeable future. In this chapter we identify the major motives underlying rural-urban migration in the Third World and examine the key structural determinants migration flows. We consider the characteristics migrant different types of populations and the migration assess the dif strategies adopted. We then policy large-scale rural-urban migration. responses to flows ranging from efforts at preventing via the imposition controls to various of migration means of accommodating the influx of population.