Thermodynamics of interfaces provides a framework to relate measurable quantities to other important yet not directly accessible equilibrium properties of interfacial systems. For liquid/gas and liquid/liquid interfaces (fluid interfaces) the interfacial tension and its dependence on temperature and composition can be measured, while the adsorbed amounts of the components are not accessible. Conversely, for solid/fluid interfaces the adsorbed amount can be measured but the interfacial tension (free energy) is not accessible. For both cases the Gibbs equation represents a bridge between the two kinds of quantities. In this chapter we explain the application of the Gibbs equation with a focus on soft matter systems. We also discuss the meaning of surface excess amounts and their relation to (absolute) surface concentrations which appear in adsorbate equations of state. Finally we briefly touch the additional features of charged interfaces and of ionic equilibria at interfaces.