The in situ MAS NMR studies of the mechanisms of zeolite catalyzed reactions are reviewed. The first part of the critical review contains brief information on the different experimental approaches used for the in situ MAS NMR studies under batch and flow conditions. In the second part, a cross reference index between the reactions studied, the catalysts used, the mechanistic information obtained and the corresponding literature sources is established. Finally, in the last part the most widely studied areas are discussed in more detail. In particular, the impact of in situ MAS NMR to unravel the mechanisms of olefin, alcohol and alkanes transformations over zeolite catalysts is analyzed (232 references).