This work presents a reverse-engineering process for legacy information systems understanding and maintenance that takes its main steps from the Unified Process (UP). We show that the reconstruction of the use-case model of the system is central to the recovery of the architecture of the system. First, the use-cases allow us to recover the model of the business process the system supports. Second, these use-cases are analyzed to rebuild the corresponding system analysis model representing the high level architecture. The latter will be used as the "hypothetical architecture" to lead the search for the corresponding software elements in the system. Third, the use-case model is used as the source of the scenarios to be run to find the software elements that implement the business functions. Then the "hypothetical architecture" can be compared to the components identified by running the scenarios. As a result, a matching can be made between the business functions of the business model and the software elements of the system. This helps the maintenance engineer to understand the purpose of these elements.