This review provides a survey of the chemistry literature on perchlorates and perchloric acid as reagents or catalysts for various organic reactions mainly in this century. Perchloric acid is potentially an explosive compound when it is heated and contacted with combustible materials. Therefore, it must be handled very cautiously, but absorption on silica gel produces great improvement in terms of handling, being inexpensive, nontoxic, available, and safe. The metal ions of perchlorates act as Lewis acids, and the Lewis acidity is dependent on the solvent basicity, since perchlorate ion has very low basicity. Perchlorate salts promote several synthetic transformations that are otherwise difficult to perform. Apart from the acceleration of the rate of the reaction, high chemo-, regio-, and stereoselectivities have been observed, as demonstrated by the many examples cited in the text. The reactions are generally carried out under very mild conditions, often at room temperature and under essentially neutral reaction and workup conditions.