ContentsI. Introduction 171 II. Acid Catalysis by Heteropoly Acids 172 A. HPA Catalysts 172 1. Structure 172 2. Acid Properties 173 3. Supported HPAs 175 4. Salts of HPAs 177 B. Homogeneous Reactions 177 1. Hydration of Olefins 178 2. Esterification and Related Reactions 179 3. Condensation 179 4. Miscellaneous Reactions 180 C. Biphasic Reactions 180 1. Polymerization of Tetrahydrofuran 180 2. Esterification 181 3. Other Reactions 181 D. Heterogeneously Catalyzed Liquid-Phase Reactions 181 1. Paraffin Alkylation 182 2. Friedel−Crafts and Related Reactions 182 3. Esterification, Hydrolysis, and Related Reactions 183 4. Miscellaneous Reactions 184 III. Multicomponent Polyoxometalates in Liquid-Phase Oxidation 184 A. Oxidation with Dioxygen 184 1. General Principles 184 2. Homogeneous Oxidation 185 3. Biphasic Oxidation 187 4. Heterogeneously Catalyzed Oxidation 188 5. Mechanism 189 B. Oxidation with Hydrogen Peroxide 191 1. Venturello−Ishii System 191 2. Other Polyoxometalate Systems 193 IV. Conclusion 194 V. Acknowledgments 195 VI. References 195 Ivan V. Kozhevnikov was born in Novosibirsk, Russia. He received his M.Sc. in chemistry from Novosibirsk University in 1969 and Ph.D. from Boreskov Institute of Catalysis (BIC) in 1972, where he studied with Professor E. S. Rudakov. After three years of postdoctoral studies at the Institute of Physicoorganic Chemistry in Donetsk, he returned to BIC and joined Professor K. I. Matveev group. In 1984 he received his D.Sc. degree and was promoted to Head of laboratory at BIC and appointed Professor at Novosibirsk University. He has held visiting positions at Lund University, Sweden (1990),