The city as a progressive system: structural evolution and economic development. By moving from the theory of ‘progressive systems’ and assigning to the causal relationship ‘structure-performances’ a key role this paper outlines a methodological perspective to study the economic development of cities. The proposed perspective leads to shift the scientific focus of urban economics to the identification of the ‘structure’ and the analysis of the ‘structural evolution’ of cities. Moreover, being cities as any other complex system an ‘open system’, its structural evolution has to be evaluated with regard to its changing ‘environment’. To study the development trajectory of city means, therefore, to study the evolution of its structure, which is the emerging outcome of the interplay of a constellation of adjustment mechanisms, each with its own rationale, which have to be examined with a substantive rather than a formal approach.