This research seeks to explain contrasting patterns of population stability and decline at both the city and neighborhood levels. Existing research on urban decline identifies several factors that can facilitate or impede population stability, including: the amount of socioeconomic disadvantage, the physical and economic characteristics of a city, local government responsiveness, and the strength of civil society. Using a mixed methods approach combining multilevel modeling and spatial analysis techniques, I test the hypothesis that stronger civil society, characterized by greater collective capacity and a denser organizational resource base, is the key determinant in whether a city will stabilize, grow, or decline. First, the analysis explores divergent patterns of population change at the city level using an original time-series, cross-sectional dataset on 231 cities over four decades. At the city level, strengthening the collective capacity of civil society is an effective way to mitigate decline. Targeting local allocations of Community Development Block Grant (CDBG) funding and reducing socioeconomic disadvantage can also stimulate population growth over time. To see how these forces work at the local level, I explore neighborhood variation in Detroit, MI. The analysis looks first at local CDBG allocation decisions and then examines the forces that influence population change in Detroit neighborhoods. Geographically weighted regression results reveal significant nonstationarity in relationships between the explanatory variables and population change. In general, stronger civil society leads to greater population stability. Local CDBG investment strategies, however, tend to weaken the positive effect of civil society with a few notable exceptions. In more racially and ethnically diverse and residentially stable areas, neighborhood organizations effectively broker resources. In these neighborhoods, the combination of a strong civil society and local government investment best predict population stability. Additionally, concentrated disadvantage consistently proves the greatest obstacle to population stability or growth at the city and neighborhood levels.