Fishes live in a world that is unfamiliar to us. Although we may make or even more advanced brief visits to this other world using a snorkel, scuba diving equipment, we can never become a part of it. Yet, an understanding of fish ecology requires an awareness of the relationships between fishes and their environment. The purpose of this book is to introduce the ecology of fishes by describing the inter-relationships between fishes and the aquatic habitats they occupy. The book can be read in complementary ways. A sequential reading, chapter by chapter, covers the main themes of ecology, including habitat use, species interactions, migration, feeding, population dynamics, and reproduction in relation to the major habitats occupied by fishes. An alternative reading selects a particular sort of habitat, such as rivers, and, by using the index and skipping from chapter to chapter, builds up a picture of the ecology of fishes living in that habitat. The text is written for advanced students. Its emphasis is on descriptive rather than quantitative ecology. It is assumed that the reader will be familiar with the basic biology of fishes, acquired from a text such as The Biology of Fishes (Bone and Marshall, 1982) also published in the Tertiary Level Biology series. I would like to thank Dr. J. D. Fish and two anonymous reviewers who, within a tight time schedule, tried to improve the text. Any mistakes and shortcomings are my contributions.