This is part 1 of the proposal. It contains the full scientific justification etc. and the category I source list. The category II and III source list is contained in part 2 of the proposal. Part 2 request another 34540 sec observing time, so that the total proposal is 72270 sec in each launch window. We propose a detailed spectroscopic study of PDRS, selected over a broad spectrum of physical parameters beyond that included in the central programme. Together with advanced PDR models that account for complex but realistic cloud topologies (e.g. clumpiness, physical state of the interclump medium, nature of the clump/interclump boundary) and for previously neglected, though often important, aspects related to the dynamic evolution of the PDR structure (e.g. location and propagation of the ionization and the dissociation front under different physical conditions), these observations can provide important information on the physics governing the interaction of UV radiation and the ISM in a wide range of astronomical objects.