Heteropoly compounds (HPCs) have been a matter of interest in basic and applied science for more than a century. From their first synthesis, many advances have been made to promote the use of HPCs in different ways in science and technology. The aim of this article is to review the main structural characteristics of heteropoly compounds of the Keggin type (12-tungstophosphoric, 12-molybdophosphoric, 12-tungstosilicic acids; alkaline and alkaline earth salts of 12-tungstophosphoric acid and gels doped with HPCs) to understand and explain their different activities, such as high proton conductivity, and catalytic, biochemical and biomedical activities. (c) 2005 Elsevier B.V. All rights reserved.