Cities exist because proximity facilitates interactions between economic agents. There are few, if any, fundamental issues in urban economics that do not hinge in some way on reciprocal action or influence between or among workers and firms. Thus, the localization of industry arises from intra-industry knowledge spillovers in Marshall (1890), while the transmission of ideas arises through local inter-industry interaction that fosters innovation in Jacobs (1969). In fact, the faceto-face interactions that Jacobs emphasizes are believed to be so critical to cities that Gaspar and Glaeser (1997) (and others) have asked whether advances in communication and information technology might make cities obsolete.