title: Proto-planetary Herbig Ae/Be discs as seen with Herschel: atomic and molecular emission lines as tracers of the disc chemistry and physics. authors: G. Meeus, for the GASPS and DIGIT teams abstract: The mechanisms determining planet formation are not (yet) well-understood. Primordial proto- planetary discs consist for 99% out of gas, and only 1% out of dust. With time, those initially flaring discs are believed to evolve into a flat geometry, when the initially small dust grains grow to larger sizes and settle towards the mid-plane in which planets can form. In the mean time, the gas will disperse, until so little is left that giant planets no longer can form. As an important piece of the puzzle of planet formation, it is important to understand 1) the influence of the heating/cooling processes on the young disc structure, 2) the chemical composition and its evolution and, finally 3) how fast gas gets dispersed, and which mechanisms control this dispersion. In this talk, I will concentrate on the proto-planetary discs around Herbig Ae/Be stars (HAEBE), young objects of intermediate mass, in the context of their gas content and properties. We compare our observations with those of young debris disc stars, for which the HAEBE stars are thought to be progenitors. We present new Herschel PACS spectroscopic observations that were obtained within the GASPS and DIGIT Open Time Key Projects (PI B. Dent and N. Evans). We concentrate on the detection and characterisation of both atomic and molecular emission lines, tracing the disc at different vertical depths between 5 and 500 AU. Our spectra cover transitions of [O I], [C II], OH, H2O, CH+ and (mid to high J ) CO, the most abundant components of the disc. We look for correlations between the observed line fluxes and stellar properties such as effective temperature and stellar luminosity, FUV and X-ray luminosities, several accretion diagnostics, as well as with disc properties: amount of flaring, PAH bands and (low J ) CO transitions, tracing the cold gas (mass). We will present a few cases to illustrate how simultaneous modeling (using the thermo-chemical disc code ProDiMo) of the atomic fine structure and molecular lines can constrain the disc gas content, once the disc structure is derived. We show the first solid detection of water and OH in a Herbig Ae disc, and discuss the presence (and absence) of these molecular lines in the context of the disc physics and chemistry.