Book excerpt: Of all vertebrates, fish exhibit unparalleled diversity of sexual plasticity and flexibility, ranging from gonochorism to unisexualism, and exceptional patterns of functional hermaphroditism. Fish farming and monosex aquaculture have led to reproductive dysfunction with males producing less milt, and females failing to ovulate and spawn. This book brings together relevant information on the role of the endocrine system on sexual differentiation in fish, and bridges the gap between molecular endocrinologists and fishery scientists.