This paper reviews and discusses the uses of genetic markers in analyzing the composition of population mixtures of fishes. Such analyses require (1) the existence of genetic differences identified in base-line data from contributing populations, and (2) an adequate sampling of individuals from the mixture to characterize its composition. Genetic analyses of complex population mixtures of salmonids began around 1980 and subsequently have been increasing; applications have included estimates of geographic origins in marine and freshwater fisheries, and of ancestral origins in populations reestablished from multiple sources. The potential for applications of genetic analyses of population mixtures is more limited in most marine species because such species are typically less genetically subdivided than salmonid species. However, genetic analyses are feasible in any species in areas of intermingling of genetically distinguishable populations. In addition, genetic marking by preferential breeding of ...