Seasonality is a common feature of teleost reproduction and is most marked at higher latitudes. Seasonal cycles are principally entrained by photoperiod and temperature but a range of other variables including lunar phase and social interaction may also operate. Photoperiod information is transduced through the light inhibition of pineal melatonin synthesis but the mechanism by which this regulates the reproductive endocrine system is not clear. There is less information on the specific effects of temperature but these are likely to involve effects on expression and activity of regulatory proteins. Environmental regulation of high latitude species appears to operate across a hierarchy of variables with photoperiod then temperature as primary drivers, whereas in tropical habitats, the hierarchy may be inverted.