The molecular sieve effect is observed in porous materials (so-called molecular sieves - zeolites, zeolites-like materials, active carbons etc.) with the pore size comparable to molecular size, as well as in cell membranes (e.g. glycocalyx on the vascular endothelial tissues). There are several types of molecular sieve effect. Selective mass transfer takes place if the diffusion rate inside pores of moleqular sieve is different for molecules of different size. Another example of the molecular sieve effect is form selectivity, where only molecules whose transition state size is smaller than the pore size can form inside the pores in the course of chemical reactions. A number of selective adsorption processes, catalysis, membrane separation etc. are based on the molecular sieve effect.