Insight into the mechanisms that control reproduction in fish is important for several reasons. In aquaculture, improved broodstock management is important to obtain good, stable gamete quality, to ensure fish welfare and for seasonally independent production of viable offspring. In on-growing farms, early sexual maturation is a major bottleneck in most species. Maturation leads to weight loss, impaired flesh quality and often increased mortality. In wild populations, reproduction can be affected by various natural and anthropogenic influences, such as water temperature and pollution. Sexual maturation is regulated through the brain-pituitary-gonad axis, and many central components of this axis have recently been identified and characterized in cod and halibut. Environmental effects, especially photoperiod and temperature, on onset of puberty, sexual maturation and spawning have been given special attention. Photoperiod treatments can be used to arrest or accelerate puberty in halibut and cod. Such treatments and responses represent useful models to investigate the molecular and endocrine mechanisms involved in puberty control. Also, Atlantic halibut have a pronounced sex dimorphism in growth pattern and age at maturity: females have higher growth rate and grow larger than males, they mature at a higher age and show a pre-pubertal increase in growth that is less marked, or absent, in males.