The neighborhood is an important geographical, functional, and symbolic unit for individual and societal wellbeing, yet neighborhood environments in the U.S. have long been characterized by racial and economic inequality and segregation. With more research linking the neighborhood with life outcomes, there has been a growing interest in creating mixed-income neighborhoods, materializing in policy changes that incentivize affordable housing investments in high opportunity neighborhoods. However, there is still a critical lack of knowledge on the phenomenon of neighborhood income diversity, which has historically been a planning ideal and is increasingly a policy goal. This dissertation addresses this gap by posing the following principal research question: What explains neighborhood income diversity, and what is the role of planning and housing policy? A series of analytical sub-questions structure the study: i) How can neighborhood income diversity be defined, conceptualized, and measured?; ii) What are the temporal trends, spatial patterns, and correlates of neighborhood income diversity?; and iii) What are the characteristics and explanatory factors of persistently mixed-income neighborhoods? Identifying a variety of measures used in the literature, this study presents a conceptual typology of neighborhood income diversity—separation, evenness, disparity, and representativeness—and its methodological implications. Leveraging statistical and spatial analyses on census tracts in the 100 largest metropolitan statistical areas (MSAs) for 1990-2019, it finds that neighborhood income diversity has increased, is a spatially clustered phenomenon, and is negatively associated with federal subsidized housing programs, though the relationship varies at the MSA level. Finally, employing a mixed methods case study approach for two cities, Philadelphia, PA, and San Francisco, CA, the dissertation finds that persistently mixed-income neighborhoods (PMINs) are heterogeneous in terms of their demographic, socioeconomic, and housing attributes and their neighborhood change trajectories. PMINs’ explanatory factors, including local planning and housing policies, vary not only between the two cities but also within them, dependent on the neighborhood context. This dissertation argues that neighborhood income diversity is a complex, heterogeneous, and dynamic phenomenon and that fostering it requires a neighborhood-sensitive approach to policy interventions.