In situ MAS NMR has been demonstrated to be a superior technique for analyzing desorbed reactants, reacting adsorbates, intermediates and products. The results are useful for getting deeper insights into catalytic processes and reaction kinetics, and for re vealing the mechanisms of organic reactions. The utility of in situ NMR spectroscopy has demonstrated that this technique is an important means for the studies of catalytic organic reactions. The objective of this paper is to give a review on the generatio n of apparatus for in situ MAS NMR experiments, and to illustrate the applications of the in situ MAS NMR techniques in various important organic reactions catalyzed by solid catalysts. The typical reactions studied by in situ MAS NMR techniques include conversion of methanol to hydrocarbons (MTG process), aromatization of methane, ethylbenzene reaction over zeolite HMOR, observation of carbenium intermediates formation, and photocatalytic organic reaction.