The gill bearing aquatic craniate animals which lack limbs with digits are known as fish. Some of the different categories of fish are lampreys, hagfish, bony fish and cartilaginous fish. Generally fish are ectothermic, which allows their body temperature to fluctuate depending upon the change in ambient temperature. The structure of a typical fish usually contains two sets of paired fins, one or two dorsal fins, an anal fin and a tail fin. Its body is streamlined and covered in scales, and it lays eggs. The various species of fish are divided into two broad groups on the basis of the ecosystems which they inhabit. These are marine, also called saltwater fishes, and freshwater fishes. This book provides significant information to help develop a good understanding of fish biology and related fields. It will also provide interesting topics for research which interested readers can take up. The book is appropriate for students seeking detailed information in this area as well as for experts.