A very substantial amount of additional information has been published concerning hydrides of the platinum group metals over the two decades since the hydrides of palladium and palladium alloys were the subject of an earlier review article in this Journal. In addition to the many articles in the general literature, the subject matter has formed a major part of the programmes of several scientific conferences and of a number of books and monographs appearing over this period. Furthermore, silver-palladium diffusion tubes are incorporated into hydrogen generators built by Johnson Matthey, and utilised for such diverse applications as the hydrogenation of edible oils, manufacture of semiconductors, annealing of stainless steel and the cooling of power station alternators. In view of the considerable interest being shown in both theoretical and technical aspects of these systems this unusually long review is presented, and will be published in parts during the year.