In this volume our aim is to highlight some of the exciting research that has emerged on molecular biology of fish hormones, their receptors, and regulation. Like studies in biomedical sciences, comparative vertebrate studies have found that molecular biological techniques are a powerful and indispensable tool for advancing our knowledge of gene structure, evolution, and regulation of fish hormones. Comparative studies of the structure of these hormones, at both the protein and the DNA level, provide important clues about the structurefunction relationship of the hormones, as well as their evolutionary history and mechanisms of action. Similarly, elucidation of regulatory DNA sequences is a prerequisite for studying tissue and celltype specificity, temporal expression ofthese hormones, and regulation by various factors.