Research into the processes leading to impoverishment in urban environments has, in recent years, given much space to the study of the final stage in the «careers» of these subjects, which can be defined as extreme poverty. However, we believe it is also necessary to investigate the steps preceding the final phase (summed up in the concept of vulnerability proposed by R. Castel). This paper therefore focuses on the phenomenon of vulnerability, which by definition privileges the dynamic component - that is, a chain of events that affect the lives of the subjects - of the processes leading to impoverishment. From this point of view it is necessary to turn our attention to methodologies concerned with the process, and we propose an investigation based the analysis of case-studies through the study of «biographical paths». For this purpose we offer some thoughts about the conceptual dimensions and the analytic paths linked to situations of vulnerability, for use in the hypothetical «biographical reconstruction» in order to give the investigation a suitable «operative» direction.