In this review, the most relevant aspects of skeletal isomerization of n-butene to isobutene are discussed : the nature of the active sites, the prevailing mechanism of the skeletal isomerization, and the relation of this to that of n-butane. It is concluded that the prevailing mechanism of skeletal isomerization ofn-butene is monomolecular (in contrast to butane isomerization) and requires Bronsted acid (OH) active sites. The selectivity and catalytic stability can be influenced by the shape selectivity of zeolites and zeotypes. These effects are explained on the basis of the knowledge on the prevailing mechanism.