ABSTRACT: In classic urban sociology, the zone of transition was an expression of the spatial logic of the industrial city, and a site of particular conflict within it. In the near-century since its introduction, metropolitan areas have been fundamentally reconfigured by processes such as suburban expansion, demographic diversification, and economic transformation. We explain how those reconfigurations have generated new zones of transition, based on a new spatial logic shaped by fundamental tensions between polarization and proximity that simultaneously push groups apart and force them together. Those tensions are concentrated in contemporary zones of transition, and most specifically in intrametropolitan border communities where competition over use is most intense and ethnic succession most active. Consequently, those communities become crucibles in which broader tensions over diversity and inequality are intensified and played out, with implications far beyond their boundaries. To illustrate our arguments, we focus on Orange County, California, which has become an exemplar of new patterns of metropolitan growth and development and a site of significant ethnic and socioeconomic diversity (notwithstanding media misrepresentations). We explain how underlying tensions and resulting unevenness generate (both geographically and occupationally) a zone of transition between relatively insulated homogeneous communities and stable multiethnic and majority–minority communities. To illustrate the concept of border communities, we focus on Costa Mesa, which has achieved national attention associated with conflicts over immigration, race, and day labor.