Theoretical studies on the effect of generation-recombination aggregation of Frenkel defects created by irradiating solids with radiation are reviewed. We discuss the way in which a substantially non-Poisson spectrum of density fluctuations of the number of defects is formed by the stochastic creation and recombination of immobile and mobile defects. The results obtained by using different approaches are compared--multiparticle densities, diagram technique, scaling, computer simulation. We show that the aggregates created in the course of accumulation of sametype defects (atoms or vacancies) substantially affect the kinetics of defect accumulation and increase severalfold their concentration at saturation at large radiation doses. We examine in detail how the spatial distribution of defects depends on their mobility, on the dimensionality of the space, on correlation in genetic pairs being created, on the mechanism of recombination (annihilation or tunneling charge-transfer), and on the number of crystal-lattice nodes in the sphere of recombination. Experimental data confirming the calculations are presented.