The reaction of aminomethylcycloalkanes with nitrous acid to produce cycloalkanols in which the ring is larger by one carbon atom is known as the Demjanov rearrangement. The first example of this type of ring expansion was encountered in 1901, but was not recognized until 1903 when cyclopentanol was identified as one of the products formed from cyclobutanemethylamine. Since that time the reaction has extended to rings of many sizes. Olefins almost invariably accompany the alcohols that are formed. The Demjanov rearrangement includes within its scope the rearrangements that occur when acyclic amines are treated with nitrous acid as well as other ring expansion detailed in this chapter. The Tiffeneau-Demjanov ring expansion is used here to designate ring enlargements by pinacolic deamination. This ring expansion can be made the key step in the conversion of a cyclic alcohol or ketone into its next ring homolog.