Surface Science has in the past provided much of the framework and tools for understanding heterogeneous catalysis. Nevertheless, the pressure and structure gap still continues to provide unforeseen and interesting phenomena, which are necessary prerequisites in order that real catalytic processes under high-pressure conditions can be understood and modeled. It is therefore mandatory to identify which reaction pathways are prevailing and what structure and composition are present at increased pressure. In a number of examples it will be demonstrated how, by combining in situ high pressure experiments on well defined single crystals with DFT calculations, it is possible to gain a further insight in this direction. For example is it well known from UHV experiments that steps or defects may influence the surface reactivity substantially, but the influence on catalytic process have been of more speculative nature. It will be shown, how steps and/or defects increases the sticking of N@sub2@ on Ru(0001) by 8 to 9 orders of magnitude!! Since this is the rate-limiting step in the ammonia synthesis on Ru, it will naturally have a profound impact on the understanding of the ammonia synthesis, which was also studied. Turning to model system for alloy catalyst it will be demonstrated how the surface composition and availability for performing chemical reaction can be strongly dependent on gas induced segregation phenomena. A brief overview of the determining parameters will be given showing that the surface composition of alloys is dynamically dependent on temperature and the chemically potential of the gasses involved and cannot in many cases be determined from UHV experiments alone.