Fish are aquatic craniate animals which lack limbs with digits but possess gills. Some of their common types are bony fish, hagfish, lampreys and cartilaginous fish. These are cold blooded animals with the exception of active swimmers like sharks and tuna. Fish communicate with each other by means of acoustic communication. This involves production of stridulatory sounds and non-stridulatory sounds. Fish generate these sounds by moving parts of their skeletons or using a dedicated organ like the swimbladder. They exist in most of the aquatic environments ranging from high mountain streams to hadopelagic zones. Fish have streamlined bodies and have two sets of paired fins. Most of them are covered with scales and lay eggs. This book presents the complex subject of fish biology in the most comprehensible and easy to understand language. Its extensive content provides the readers with a thorough understanding of the subject. This textbook is appropriate for students seeking detailed information in this area as well as for experts.