The discovery and study of the biological action of substances possessing hormonal properties comprise a considerable step towards a more comprehensive illustration of fish physiology. Fish in their vast majority do not constitute merely the most populous and phylogenetically oldest vertebrate group of organisms, but also a constant source of information regarding the mode of action of physiological and endocrinological processes of the phylogenetically evolved vertebrates, including mammals and humans. Although cells with neurohormonal properties have been identified in fish during the first decades of the 20th century, it is only over the past thirty years that these cells have been the object of intensive research efforts which, at an ever accelerating pace, are attaining a number of significant goals. This book presents current research in the study of fish endocrinology.