We are interested in designing a reusable and robust activity monitoring platform. We propose three good properties that an activity monitoring platform should have to enable its reusability for different applications and to insure performance quality: (1) modularity and flexibility of the architecture, (2) separation between the algorithms and the a priori knowledge they use, and (3) automatic evaluation of algorithm results. We then propose a development methodology to fulfill the last two properties. The methodology consists in the interaction between end-users and developers during the whole development of a specific monitoring system. To validate our approach, we present a platform used to generate activity monitoring systems dedicated to specific applications, we also describe in details the technical validation and the end-user assessment of an automatic metro monitoring system built with the platform and briefly the validation results for bank agency monitoring and building access control.