Surface Science UHV studies, in particular when concerned with metals or metal oxides, in some sense often investigate an artificial state. This means, the surface stoichiometry, and as a consequence the electronic properties, of the studied systems are significantly different to those that would exist in a realistic environment, which typically contains oxygen and water at higher pressure and temperature. This talk summarizes concepts for describing how the pressure and temperature of the environment or atmosphere affect surfaces.@footnote 1-3@ --- I will also describe recent developments in methodology: By combining density functional theory calculations with a lattice-gas Hamiltonian approach and the kinetic Monte Carlo method, surface phenomena can be described over time scales of the order of seconds and length scales of the order of micron.@footnote 4,5@ Examples of such calculations consider long-ranged adsorbate-adsorbate interactions,@footnote 5@ island formation and crystal growth, as well as surface structures with disorder, and thermal desorption.@footnote 4@ --- Using Ru as the main example (but also mentioning Al, Cr, Fe, Rh, Ag) I will show that the interaction with an oxygen atmosphere gives rise to a dramatic restructuring of the surface, yielding to mesoscopic domains of different stoichiometry and very different chemical activity.@footnote 1, 6-8@ The results will be discussed in view of their possible relevance for oxidation catalysis. --- @FootnoteText@@footnote 1@X.-G. Wang et al., Phys. Rev. Lett. 81, 1038 (1998). @footnote 2@ X.-G. Wang, A. Chaka, and M. Scheffler, Phys. Rev. Lett. 84, 3650 (2000). @footnote 3@ X.-G. Wang and M. Scheffler, to be submitted to Phys. Rev. B. @footnote 4@C. Stampfl et al., Phys. Rev. Lett. 83, 2993 (1999). @footnote 5@K.A. Fichthorn and M. Scheffler, Phys. Rev. Lett., May (2000). @footnote 6@A. Böttcher, H. Conrad and H. Niehus, J. Chem. Phys. 112, 4779 (2000). @footnote 7@ A. Böttcher et al., submitted to Surf. Sci. Lett. @footnote 8@H. Over et al., Science 287, 1474 (2000).