The thermodynamic method is one of the most fruitful approaches to the study of interfaces. After the pioneering work of Gibbs on the adsorption equation, Guggenheim considered the interface as a phase having thickness, which allowed Randles to find the relation between the bulk and the adsorption activities. From the electrocapillary equation and the Randles relations, the interfacial electrochemical potentials of neutral compounds and ions and the electric equations of state are obtained and applied to the study of phase transitions and to the computation of capacity curves. The electrocapillary equation for binary electrolytes evidences that the Gouy-Chapman theory, which applies the Boltzmann statistics to ions in the diffuse layer, is incompatible with bulk activity coefficients distinct from the unity. The compatible theory, a correction tending to the Gouy-Chapman theory at dilute solutions, overcomes this inconsistence. Rice modelled the polarization of the metal by means of the Fermi statistics. The incorporation of the work function to the Rice model explains the correlation found between potentials of zero charge and work functions for several metals. With regard to ionic co-adsorption, the electrocapillary equation at constant ionic strength yields directly the specific surface excesses of both co-adsorbed ions, and a pair of co-adsorption isotherms is deduced from the configurational entropy. Thermodynamic models have repeatedly shown their capability to predict interfacial behaviours and to calculate interfacial magnitudes, and they also offer us clear interpretations of the interfacial phenomenology. There also exist thermodynamic restrictions to the free choice of theories that help us to discard inconsistent models. Here, all these applications and implications of thermodynamics are reviewed.