A review on the iminium ions reactions where it is being active that is related to the catalytic cycle is presented. A focus is given on the synthetically useful protocols and small-molecule catalysts while the Knoevenagel and Mannich reactions have also been reviewed. An overview on the history of the iminium catalysis is given to show its evolution over the years. Then, the structural requirements of iminium catalysis were discussed. Lastly, an illustration were given on the individual reactions in the following order: cycloadditions, conjugate addition reactions, reactions involving both iminium and enamine intermediates, domino processes, and reactions with saturated compounds.