The experimental as well as the theoretical methodology for the determination of the properties of adsorbates on simple metal surfaces are well developed by now. This does not mean that there are no ambiguities and uncertainties, but the understanding of static and even dynamic properties in simple cases has reached a level where it makes sense to compare theory and experiment with the aim to get to a reciprocal improvement and an overall improved understanding.

Here I will review a number of cases of adsorbates on the hexagonally close-packed Ru(001) surface as a prototype transition metal surface, for which high quality, state-of-the-art data and calculations exist. It should be stressed from the start that this surface itself shows little catalytic activity, but it can serve to demonstrate the points made above. The simplest adsorbate for which at the same time the best agreement of theory and experiment is found is oxygen which can exist in four well-ordered structures up to monolayer coverage. For the geometries and even for a subtle effect such as the shifts of the core level binding energies of the surface Ru atoms ("SCLS") in these various phases excellent agreement of experiment and calculation has been obtained. The restructuring of the Ru surface layer (in this case mainly expansion of the first layer distance) is intimately connected with electron redistribution. Interestingly, cause and effect can be turned around: If the surface is expanded or contracted, the adsorption energy is locally changed as can be demonstrated by experiment. Again, agreement with calculations is quite reassuring. However, catalytic activity (e.g. for CO oxidation) is very low in all those cases, and recent results by Over and coworkers indicate that high reactivity is brought about by much more massive O uptake and the accompanying restructuring.

Molecular adsorbates (CO, NO) and their coadsorption with O atoms show a wide range of structures which contain some systematics. Here calculation is more difficult, and it is maybe not surprising that agreement with experiment is not so impressive. Interesting restructuring effects by coadsorbate interactions are found in the system (O+NO), and while first principles calculations do not exist here yet, the findings can be nicely described on the basis of statistical thermodynamics of lateral interactions. Real disagreement is found for a weak adsorbate such as H2O where theory seems to point to a simpler situation than experiment finds. It appears that weak bonding is most difficult to render in theory. A kind of detective story has been unfolding recently in the case of the first layer(s) of water which will be described in some detail. The general question of whether H atoms can be seen in LEED is addressed in connection with some coadsorbate systems.

In this author's view we are at a very interesting point in Surface Science where the maturity of this field leads to a fruitful competition between experiment and theory, and allows to address more complicated questions.