There are two major avenues for future research. First, we need more attention to be paid to exogenous sources of variation (instrumental variables. All explanatory variables have the problem that they are exogenous and quite potentially correlated with other variables that may truly be driving growth. The second primary avenue for future research is to try and measure the localization of ideas more effectively. Jaffe et al. (1993) is the paradigmatic paper in this area which uses patent citations to document that ideas move only slowly over time. Audretsch and Feldmann (1996) also use an innovative data set measuring new product innovations to inform us about idea production in cities. New variables and data sets such as these will be invaluable in understanding the processes that drive urban growth.