Studies on the relationship of immigration on health often find that immigrants are healthier compared to their United States (U.S.)-born counterparts despite having lower socioeconomic attainment, a finding known as the “immigrant health paradox”. However, this health advantage declines over time. Two popular theories have been proposed as explanations for this advantage: migrant selectivity and acculturation. Migrant selectivity suggests that those who immigrate to the U.S. are not representative of their host country. Instead, immigrants may be positively selected upon social, socioeconomic, and health factors that allow for them to thrive upon arrival to the U.S. and give them the observed health advantage compared to U.S.-born residents. Acculturation theory suggests that immigrants’ health declines because of exposure and adoption of U.S. culture and values.Although these theories continue to remain popular, there remain two serious methodological issues to consider. First, in order to claim that migrant selectivity occurs, it would be necessary to compare immigrants to those who did not immigrate – non-migrants from the sending country. Thus, non-migrants serve as a more ideal counterfactual to evaluate the effects of migration on health. However, there remains a dearth of studies making this comparison because of the difficulty in simultaneously recruiting recent immigrants to the U.S. and their corresponding sending country counterparts. Second, there are also few longitudinal studies examining the effects of acculturation on health. Most studies are cross-sectional and rely on proxy measures of acculturation (e.g., English proficiency and years in the U.S.). Although these measures are useful, it is difficulty to disentangle whether changes in health are indeed related to acculturation or if they are due to secular effects. This dissertation uses the baseline, 1-year, and 2-year waves of the Health of Philippine Emigrants Study (HoPES, n = 1,637) to 1) examine if migrant selection for health, social and socioeconomic factors occurs prior to migration; 2) track how migrant health, social, and socioeconomic profiles change up to two years after migration; and 3) evaluate the extent that changes in social and socioeconomic factors explain changes in health for migrants post-migration. Overall, I found that migrants had lower levels of psychological distress and sleep disturbance compared to non-migrants both before migration, even after accounting for demographic, social, and socioeconomic factors. However, there was little evidence to suggest that migrants had lower allostatic load compared to non-migrants before departure. Furthermore, I found that psychological distress and sleep disturbance declined for both migrants and non-migrants through 2-year follow-up. However, migrants maintained a health advantage relative to non-migrants over time. Moreover, I found that migrants also had lower financial strain, fewer experiences of interpersonal discrimination, and higher social resources compared to non-migrants both before and after migration. Finally, I found that changes in financial strain, interpersonal discrimination, and social resources over time somewhat explained changes in psychological distress and sleep disturbance over time. Changes in these social factors over time did not differ between migrants and non-migrants. This dissertation builds upon the limited work examining immigrant health before migration as well as examining immigrant health longitudinally. This dissertation also provides a new theoretical examination of immigrant integration by examining how migrants change relative to their non-migrant counterparts. These results also reiterate the importance of migrant selection as a factor contributing to health, social, and socioeconomic advantages for migrants both before and after migration. Finally, although these social and economic factors partially contribute to changes in health over time, the finding that migrants do not differ from non-migrants over time emphasizes that changes in migrant health over time may be the result of secular effects, rather than acculturation.