Ichthyology is a branch of zoology which is concerned with the study of fishes, their biology, structure, organs and discovering their species. It includes species like chondrichthyes, the cartilaginous fish, jawless fish, agatha, and bony fish osteichthyes. There are approximately 33,400 species of fishes being studied under ichthyology. This book aims to shed light on some of the unexplored aspects of this field. Such selected concepts that redefine ichthyology have been presented in this text. It unfolds the innovative aspects of this area which will be crucial for the holistic understanding of the subject matter. Those in search of information to further their knowledge will be greatly assisted by this textbook. Ichthyology is commonly defined as 'the study of fish' or 'that branch of zoology dealing, and provides wealth of information on the strategies that fish adopt with respect to waters with markedly different physical and chemical characteristics. It shows how their physiology, behaviour and lifestyles are adapted to exploit particular niches and gives comprehensive insight into fish life under extreme conditions. The readers are introduced to the ways in which fish exemplify many phenomena of general biological interest - the existence of competitors, chaos, and predator-prey interaction. Fish pathology as well as the components of the immune system are addressed.