Fishes are a fascinating group of animals, known for their sexual diversity and plasticity. Their indecisive mechanism of sex determination and fl exibility in sex differentiation are the subject of numerous studies and publications. Sex determination is an event, may be a special one, that lasts for minutes and seconds, but sex differentiation that follows sex determination is a life long process. They are intimately interconnected and many biologists have treated them together (e.g., Devlin and Nagahama, 2002; Morinaga et al., 2004), some have even referred one for the other. With the advent of molecular biology, this area of sex determination and differentiation has become a ‘hot spot’, for molecular biologists, who are in search of sex determining genes and publish in high profi le journals, and fi shery biologists, who are interested in monosex aquaculture and are hindered with conventional fi shery biology journals. One objective of this book is to bridge the widening gap between them.