In

this article we review evidence from studies of fish, amphibians, reptiles, birds and mammals which bears on the question of whether differences in sexual behaviour are reflected by differences in central nervous system (CNS) structure. Neural structures in fish demonstrate the existence of both inter- and intra-sexual dimorphisms related to dimorphic behaviours, as well as environmentally triggered changes in the size of neural structures. Seasonal changes in neural structure in amphibians have demonstrated a strong correlation between sexually dimorphic brain structures and sex-specific behaviours. While in reptiles there are some examples of sexually dimorphic CNS structures, C.uniparens demonstrates that differences in brain morphology are not necessary to display sexually distinct behaviour. Birds demonstrate the clearest sex related brain-behaviour differences; the song control nuclei exhibit substantial differences in size between the sexes varying in magnitude in relation to the amount of sexual dimorphism in song production. There are sexually dimorphic areas in the mammalian brain, in areas associated with sexual and maternal behaviour, which  are correlated with differences in hormonal environments during ontogeny. No single phyletic trend is obvious, though this could be the consequence of a small number of taxa examined or the different aims of the studies. It appears that sexuality has not necessarily evolved linearly from a particular primitive vertebrate ancestor but is manifested variously in different vertebrate classes, most likely as the result of distinct environmental pressures.