The chapters in this dissertation each look at some aspect of immigration or internal migration in the United States, highlighting the spatial nature of population distribution and mobility. Chapters 1 and 2 focus on the effect of immigrant residential clustering on crime and Chapter 3 explores the internal migration behavior of Puerto Ricans. In the first chapter, we investigate the effect of immigrant concentration on patterns of homicide in Los Angeles County. We also suggest an alternative method by which to define immigrant neighborhoods. Our results indicate that immigrant concentration confers a protective effect against homicide mortality, an effect that remains after controlling for other neighborhood structural factors that are commonly associated with homicide. Controlling for the spatial dependence in homicides reduces the magnitude of the effect, but it remains significant. Chapter 2 examines how foreign born population concentration impacts homicide rates at the county level. This chapter utilizes a longitudinal study design to reveal how changes in the immigrant population in the county are associated with changes in the homicide rate. The analysis is carried out using a spatial panel regression model which allows for cross-effects between neighboring counties. The results show that increasing foreign born population concentration is associated with reductions in the homicide rate, a process observed most clearly in the South region of the United States. In Chapter 3 we explore the internal migration patterns of Puerto Ricans in the United States, comparing the migration behavior of individuals born in Puerto Rico to those born in the United States. Second and higher generation Puerto Ricans are more mobile than their first generation counterparts, likely an outcome of the younger age structure and greater human capital of this former group. Puerto Ricans born in the United States also appear to be less influenced by the presence of existing Puerto Rican communities when making migration decisions. Both mainland- and island-born Puerto Rican populations are spatially dispersing, with the dominant migration stream for both groups being between New York and Florida.