The formation of N-substituted amides by the addition of nitriles to alkenes in the presence of concentrated sulfuric acid was first described by Ritter in 1948. The reaction has been extended to the addition of nitriles to a wide variety of compounds capable of forming a carbonium ion, and it constitutes the only really useful procedure for the preparation of amides of tertiary carbinamines. In its most general form, the Ritter reaction involves the nucleophilic addition of a nitrile to a carbonium ion in the presence of sulfuric acid. Attention has been given to reactions in which the carbonium ion is generated in sulfuric acid, or less commonly in sulfonic acids, phosphoric acids, or boron trifluoride. For comparison, analogous reactions, which are initiated by a nitrilium ion, formed from a nitrile and a Friedel-Crafts catalyst, have been included in this chapter.