Dipartimento di Pianificazione Università Iuav di Venezia* (Paper first received, November 2006; in final form, May 2007) From agglomeration to milieu: the interpretation of city in urban economics, starting from textbooks published in Italy. With the introduction of relational goods among the factors of development, the boundaries between applied economics and other social sciences are notably weakening. This process implies the redefinition of numerous categories, particularly in the field of spatial economics. But, in doing so, it faces considerable difficulties due to the arduous insertion of sociological categories in microeconomic analysis. “City” is an emblematic example of such an issue. Alongside its interpretation derived from the neoclassical paradigm, which is formally impeccable but poor in content, a new interpretation is now coming on stage, derived from the theory of milieus. Even if it is very promising on the substantive level, it still appears to be incapable of fully integrating itself within the standard economic analysis. This paper puts forward the possibility of integrating these two interpretations, by pointing to learning as the suitable operator for its realization. Placed on this analytical background, city can be interpreted as a cognitive milieu, having original and economically relevant characters compared to analogous spatial formations.