Heteropolyanions are polymeric oxoanions which are formed by the condensation of more than two different oxoanions. Characteristics of heteropoly compounds as solid acids and solid acid catalysts have been reviewed, placing stress on the relationships between those properties and the constituent elements and on the pseudo-liquid behavior. The heteropoly compounds described in this review are mostly those having the Keggin structure. They are thermally stable and rather easily obtained. Principal merits of heteropoly compounds when they are used as solid acids or solid acid catalysts are discussed.