The Orion Bar as the canonical high-flux PDR is examined. In addition to a detailed description of the source, the estimation of physical parameters such as ionization fraction and observational indicators such as carbon recombination lines are considered. High-resolution observations point to the sensitivity of carbon chemistry to CR ionization and the apparent merging of C/C+/CO transition and H/H2 transition zones not readily predicted by theory. A wide range of molecular sulphur observations also presents the opportunity to rethink gas–grain reaction networks and model their consequences, with a following chapter looking at the low-flux PDR case of the Horsehead Nebula, through which the sulphur question will be further explored.