The literature describing the biocompatibility of glass-poly(alkenoate) ('Glass-Ionomer') cements has been reviewed. This literature shows that these materials have generally good biocompatibility for both dental and orthopaedic use, this latter observation being very recent. There have, though, been a few reports showing that in certain circumstances these materials may cause pulpal irritation and the reasons for these particular findings are considered. Following discussion of the biocompatibility of Glass-Ionomer cements, consideration is given to the likely underlying causes of this feature. Three factors are identified as contributing to the biocompatibility of these cements. They are: (i) minimal exotherm on setting; (ii) rapid neutralization following mixing; and (iii) slow release of ions which are generally biologically beneficial or, at least, benign. This last point is considered in some detail. Previous studies of leaching of ions from Glass-Ionomer cements have shown that only inorganic species are released. The biological effects of each of these inorganic ions are described and their influence on biocompatibility discussed.