Acidification of freshwaters have inflicted a major perturbation on Scandinavian aquatic ecosystems as indicated by severe regional loss of fish populations. This decline was first noted in the early 1920s but became particularly severe after World War II in the 1950s and 1960s. In southern Norway regional damage is now documented in an area of 33 000 km<latex>$^2$</latex>, 13 000 km<latex>$^2$</latex> of which are devoid of fish. Several major southern salmon rivers are now barren. In Sweden more than 2500 lakes are documented to be affected. This corresponds to 3-4% of the total lake surface area. An additional 6000 lakes are assumed to be affected by acidification. Population losses are also found in thousands of kilometres of running water as well as in salmon and seatrout rivers on the southwest coast. This paper describes the early observations, chronology of this decline and reviews possible causes and mechanisms. The acidification and the associated loss of fishstocks over vast areas is apparently the most devastating change recorded for the fish fauna of Scandinavia.