Research on the molecular aspects of fish reproduction has progressed swiftly over the past few years. With the availability of wide-ranging molecular tools, fish researchers have elucidated many of the molecular mechanisms regulating reproduction which operate in the brain, pituitary and gonad. This research has revealed novel variants of reproductive hormones and their receptors, and has shed new light on the mechanisms through which many of these genes can be activated. Several of the findings, which are reported in this book, have formed the basis for subsequent mammalian research and will also constitute the platform on which new approaches to reproductive management in aquaculture can be developed.