The rural-urban migration pattern in Colombia during the last 25 years has resulted in a population increase in urban areas from 30 to 52 percent of the total population. This study explores the causes of internal migration. Migration rates are estimated for various groups in the population to clarify who migrates and to where. A model of interregional migration is set forth and estimated for a sample of Colombian municipalities, from which we can infer the responsiveness of migration to some economic, demographic, and political developments in the rural and urban sectors of the society. Various tables pertaining to trends in rural migration rates from 1951-64 and a data appendix are included. (Author/RL)