ABSTRACT The field of spatial economics has made enormous progress in theorizing and measuring agglomeration effects, trade costs, and urbanization. Typical models establish structural determinants by making strong assumptions about which forces are relevant and how these forces interact. But many of these assumptions, about firms, agents, spatial costs, and market structures, are questionable. As a result, the field has a long way to go to establish causality, and to be able to account for spatial economic dynamics.