In Chapters 1-3, the thermodynamic basis was discussed as well as the experimental opportunities to study the interfacial properties of surfactants at liquid interfaces. The description of the various experimental protocols has shown that there is no need only for one general model to describe the adsorption process at a fluid interface. On the opposite, in most cases, the measuring principles of most of the methods suitable for studies of the interfacial dynamics require quite sophisticated boundary and initial conditions in order to get a quantitative description of the experimental data. In any case, the thermodynamic baseline is the most important prerequisite for any modeling as it represents the equilibrium state for a certain adsorption layer, that is, the target for any ongoing adsorption or relaxation process. Moreover, in many cases, the thermodynamic basis is necessary to describe local equilibrium in microscopically small parts of in total nonequilibrium macroscopic systems, for example, local equilibrium between the surface and subsurface layer in the case of diffusion-controlled adsorption.