Abstract There are few topics in colloid chemistry that have experienced such extensive investigation as that of the structure of gels. Numerous hypotheses have been presented and supported by experimental data of one character or another. Certain aspects of gels have seemed to classify them as solid solutions, others as emulsions, while yet others give strong support to a two-phase, solid-liquid structure. Although the latter theory in similar form to that postulated by Nägeli in 1858 has been accepted by most of the foremost colloid chemists, the evidence has not been sufficiently unequivocal to convince all. The evidence presented in the results of diffusion experiments through gels, the fact that the electrical conductivity, refractive index, and vapor pressure before and after setting are identical, at least in certain cases, and the known facts of syneresis would seem to leave little room for doubt of the two-phase nature of gels in general. A theory in order to be perfectly acceptable must, however, enable verifiable predictions to be made from it.