The population balance formulation has been employed in order to describe zeolite nucleation and growth from precursor gels. Emphasis is given on the role of gel microstructure in crystallization kinetics. Gel dissolution and nucleation are treated as interfacial phenomena taking place at the boundary of the amorphous solid gel and the surrounding solution. The random capillary model was used to describe the gel structure and its evolution. The simulation results capture the essential features of zeolite crystallization phenomenology and are in qualitative agreement with experimental results. The often observed maximum in the nucleation rate at the early stages of gel to zeolite transformation under constant supersaturation is attributed to the increasing interfacial area during gel dissolution. Changes in the kinetic parameters (e.g. nucleation rate constant) considerably affect the overall crystallization kinetics and the final particle size distribution. Variation of the gel microstructure can have the same order of magnitude effects on the overall crystallization kinetics.