Decades of research have paid extensive attention to the health outcomes of immigrants and the implications of their social relationships. Scholars have yet to fully appreciate, however, how immigrants' personal relationships are fundamentally shaped by the structural conditions that encompass their lived experiences, and by the temporal dynamics that unfold these experiences over time. This dissertation investigates how structural and temporal conditions contribute to the consequences of immigrants’ social relationships for their physical and mental health. The first chapter summarizes the empirical evidence on immigrants’ social relationships and health outcomes, outlining a theoretical framework on the roles of structural and longitudinal factors in underlying these patterns. The second chapter leverages data from the Mexican Migration Project to examine how gender inequalities in the sending context contribute to heterogeneity in immigrant health selection among Mexican immigrants. Analyses find that less empowered female immigrants – especially those without documentation -- saw significantly lower degrees of health selection. By contrast, male immigrants were positively selected on health even when they were relatively disadvantaged. The third chapter investigates the mental health implications of instability in immigrant adolescents’ same-sex best friends, and how it explains the association between acculturation and depressive symptoms. Using data from the National Longitudinal Study of Adolescent to Adult Health, analyses find that friendship instability was associated with higher depressive symptoms only among immigrant adolescents with a low level of acculturation. The fourth chapter examines the mental health disparities between undocumented Mexican immigrants and individuals of other race-nativity backgrounds, and how the experiences of social isolation and inadequate emotional support explain these disparities. Leveraging data from the National Health and Nutrition Examination Survey, analyses find that undocumented Mexican immigrants saw lower levels of depression than other race-nativity groups. Meanwhile, they reported both a lower risk of social isolation and a higher risk of inadequate emotional support than non-Hispanic Whites, which explained their mental health advantages in different directions. The fifth chapter discusses the substantive implications of the three research chapters for immigrants’ well-being, identifies the challenges and limitations encountered in these studies, and explores the directions of further research.