The first major histocompatibility complex (MHC) molecule in fish was identified in 1990 using a polymerase chain reaction strategy with degenerate primers (thereby avoiding the problem of low sequence similarity between higher vertebrates and fish). Since that time, MHC in fish has attracted much attention, partly due to evolutionary aspects and partly because of the potential practical consequences for future fish breeding. Knowledge of the MHC molecules in fish has thus been growing extremely rapidly. All current data point to a functional MHC system in fish. There seem to be many loci and many alleles for both classes of MHC molecules, but a lack of knowledge about functional loci versus pseudogenes and non-classical loci is making it difficult at present to establish the exact number found within each species. Fish create a few surprises, such as the lack of a link between class I and class II regions in at least two teleost species. Data on other molecules which are physically or functionally linked to the MHC genes are currently emerging. To date, no studies have established the functional implications of the MHC molecules in fish but, considering the molecular data available, such an achievement will undoubtedly occur soon. Fish may well offer a unique opportunity for identifying correlations of MHC with resistance or susceptibility to diseases.