Ab initio thermodynamics and first-principles microkinetic simulations have become standard tools in research on model catalysts. Complementing dedicated in situ experiments these techniques contribute to our evolving mechanistic understanding, in particular of a reaction-induced dynamical evolution of the working catalyst surface. This topical review surveys the methodological foundations and ongoing developments of both techniques, and specifically illustrates the type of insights they provide in the context of in situ model catalyst studies. This insight points at substantial deviations from the standard picture that analyzes catalytic function merely in terms of properties of and processes at active sites as they emerge from a crystal lattice truncation of the nominal catalyst bulk material.