The economic literature on migration has a strong focus on labor migration. It typically distinguishes between migration within countries and between countries and it focuses on the determinants of migration rather than their consequences. Motives and consequences of migration are difficult to separate. We explore the policy implications and empirical support of six common theories. We distinguish between theories of (1) the initiating causes of migration and (2) the self-perpetuating causes of migration, i.e. how current migration flows can cause future migration flows. Our discussion reveals the complexity of motives to migrate, which highlights the necessity of viewing different theories as complementary rather than contradictory. For instance, the new economics of labor migration complements neoclassical theories by emphasizing that households rather than individuals often make migration decisions and by dropping the assumption that individuals are risk-neutral. Further, macroeconomic theories complement microeconomic theories by highlighting that basic structural characteristics of the economy, such as segmented labor markets and scarcity of land, can induce migration flows. Finally, we show that when evaluating different theories empirically, there is support for different theories at different time horizons (the short run, medium run and long run), a supposition commonly ignored in the empirical literature.