Synopsis Research into the structure and function of fish has made phenomenal progress in the last two decades. Such has been the force of accumulation of new information that it has become difficult to keep abreast of the developments in the different disciplines of fish biology. My attempt to integrate some of these information on the structure and function of fish and to present them in a coherent account like this is, thus, not only customary but also a compelling and urgent need. This publication is a collection of 17 research papers and review articles on selected topics written by leading experts in the field of fish biology. The write-ups present on overview of the subject and reflect the current trends of fish research at the global level. Some of the research articles offer new interpretations while others bring out the need for a fresh look into the subject. Efforts have been made to gather information on a remarkably wide range of biological adaptations to diverse habitats and ways of life among fishes and to put them in a cladistic framework.