We investigate the determinants and consequences of increased school choice by analyzing a 22-year school panel matched to county-level demographic, economic, and political data. Using an event-study design exploiting the precise timing of charter school enrollment change, we provide robust evidence that charter enrollment growth increases racial and especially socioeconomic school segregation, a finding that is partially explained by non-poor students' transition from the private to public sector. Charter growth drives public sector incorporation, while also increasing within-public sector segregation. To assess the effects of disparate choice policies on segregation, we replicate this analysis for magnet schools, which have admissions practices intended to increase diversity, and find no evidence that magnet enrollment growth increases segregation.