During the first World War, the dramatic destruction of Belgian cities encouraged intellectuals to question the nature of urban vitality. This paper aims to depict how this intellectual quest has led to the epistemological construction of the discipline of urbanism.In a second part, the essay researches how this theoretical process undergoes a conceptual upheaval that result from the industrialization and urbanization of Belgian cities in the 1920s. The paper analyzes the methodology and tools drawn together by Belgian urbanists in order to define the concept of the industrial agglomeration.Through this historical study, the paper exemplifies the originality and coherence of the theoretical background of Belgian urbanism initiated by the members of the Society of Belgian Urbanists (SUB) and the Belgian Section of the International Congresses of Modern Architecture (CIAM). To reassess this epistemological construction of urbanism seems specifically relevant in the approach to contemporary environmental and urban problems.