This chapter begins with an overview of the subfield’s initial quest to explain the selection of healthy migrants and the post-migration deterioration in health status often seen with greater duration of residence. It turns to research that has documented the heterogeneity in migrant health patterns, moving from questions solely about proximate risk factors–such as health behaviors and individualistic selection processes–toward more upstream determinants of migrant health trajectories. There is evidence that changes in health behaviors play a role in post-migration duration effects. One of the most significant sources of variation in patterns of migrant health is origin country context. It is impossible to fully understand the selection of migrants without considering the immigration policies that create barriers or pathways to movement between various contexts. Many immigrant groups encounter racialized discrimination as they navigate ethno-racial hierarchy of their new countries. Rates of global migration are projected to increase due to climate change and accompanying social unrest.