The van der Walls theory of interfacial structure, and its later extension to include non-classical critical-point exponents, are reviewed. The capillary-wave theory of interfaces is outlined, and the attempts to reconcile it with the traditional view of interfacial structure that derives from van der Waals are discussed. Several measures of intrinsic interfacial thickness not affected by the capillary-wave divergence and not equal to the coherence length of density fluctuations are mentioned. The wetting film found by Moldover and Cahn and the recent measurements of its thickness are discussed, as are the recent theoretical and experimental studies of the liquid–vapour interface near the critical solution point of a binary liquid mixture. It is shown how the ideas of the van der Waals theory have been extended to treat the line in which three phases meet, and to determine, in particular, its composition profile and tension.