This chapter describes the theoretical development of theoretical perspectives designed to explain how economic variables impact migration. Macro theories that viewed the differences between any two nations as providing the attraction to move were initially dominant. They were followed by micro theories that emphasized how migration was driven by individuals’ cost-benefits analyses. Most recent is the New Economics of Labor Migration (NELM) which focuses upon households as the decision-making unit. This chapter also examines migrant selectivity: the way people's educational, occupational and other background characteristics predispose them to migrate. Selectivity issues are shown to complicate analysis of migrant adjustment in their destination nations.