This study examines the relationship between neighborhood structure, violent crime, and concentrations of gang members at the neighborhood level. We rely on official police gang list data, police crime data, and two waves of decennial census data characterizing the socioeconomic and demographic conditions of 93 neighborhoods in Mesa, Arizona. Although we find positive linear associations between gang member concentrations and indicators of economic deprivation and social and familial disadvantage, the results of nonlinear models reveal that at extreme levels of disadvantage, the magnitudes of these positive associations are substantially reduced. In addition, although we find that neighborhood crime has no influence on concentrations of gang members net of other neighborhood characteristics, our results reveal that neighborhood instability is a key component for understanding variability in the gang phenomenon. More specifically, our results suggest that gang membership is less likely in social contexts characterized by either a residentially unstable population or rapidly changing structural conditions. (Contains 23 notes, 3 tables, and 1 figure.)