Because the crystals in any one‐growth system grow competitively, nucleation control is one of the keys to the practical exploitation of the gel‐ growth technique. The paper discusses some of the principal factors on which nucleation in gels depends and shows how nucleation can be controlled by programming of the reagent concentrations, by trace impurities, and by varying the gel structure. Multiple reseeding procedures are described which permit the growth of larger crystals than can be obtained with single growth stages.