In many aquaculture species, one sex performs better than the other in culture (e.g. in terms of growth rate or age at maturation), or the presence of both sexes can lead to unwanted reproduction. For some species, such as tilapia and rainbow trout, techniques to produce only one sex have been developed and are now widely used in aquaculture. In some of the newer aquaculture species (e.g. halibut and cod), similar techniques could be used to overcome emerging problems. Fish are also interesting models for the evolution of sex determination, since they display a wide variety of sex determining mechanisms and where sex chromosomes are present, these appear to be in the early stages of differentiation. We use a wide variety of techniques to analyse and manipulate sex ratios and maturation in fish, including genomic analysis of male and female fish, advanced karyotype analysis techniques (e.g. FISH, chromosome microdissection), chromosome set manipulations, hormonal sex reversal and triploidy.