This volume, based on the course curricula at Degree and Postgraduate Levels of autonomous colleges, universities, and central institutes teaching fish and fisheries, has been prepared keeping in mind that fish occupies a special position in the hierarchy of animal life because higher vertebrate forms evolved from them. It is unanimously accepted that all-round understanding of histology, physiology, molecular biology, genetics, breeding methods, and existing laws are essential if we are to preserve, enhance and improve the utility of our aquatic resources.