A wide class of aerogels starts from solution of monomers in which the monomers react, forming oligomers, polymers, particles and eventually a spanning cluster or a solid network embedded in a solution: a wet gel. Meanwhile,the two classical aerogels prepared in this way are the silica and resorcinol-formaldehyde ones. In the first section, silica aerogels, silica being the most often used precursor, are treated: the reaction between them in a solution, hydrolysis and polycondensation, the growth of fractal and compact structures, their gelation and ageing after the gel point has passed. Finally, the chemistry of silica aerogels with lower functional silanes is briefly discussed. In the second section, the chemistry of resorcinol (R) and formaldehyde (F) is presented, as well as the reaction between both molecules under basic and acidic conditions and how polymers develop from monomers. The effect of various process parameters, the ratio of R to F or the concentration of a catalyst, the dilution ratio with water and the influence of temperature on gelation are treated in detail. Finally, some thoughts on the thermodynamics of RF gels are presented.