Inner-city crime is a motivating factor for middle-class flight. Therefore, crime is a cause of suburbanization. Movement of the middle and upper classes to the suburbs, in turn, isolates the poor in central-city ghettos and barrios. Sociologists and criminologists have argued that the concentration of poverty creates an environment within which criminal behavior becomes normative, leading impressionable youth to adopt criminal lifestyles. Moreover, from the perspective of routine activity theory, the deterioration of social capital in high-poverty areas reduces the capacity for guardianship. Therefore, suburbanization may also cause crime. The authors argue that prior research has not distinguished between the causal and compositional effects of suburbanization on crime. They show that the causal component can be identified by linking metropolitan-level crime rates, rather than central-city crime rates, to measures of suburbanization. Using Uniform Crime Reports and census data from 2000, they find a positive relationship between suburbanization and metropolitan crime. (Contains 1 figure, 4 tables and 4 notes.)