Dive into the fascinating world of fish biology with this introductory ichthyology lecture. Explore the fundamental concepts of fish classification, evolution, and diversity. Learn to define what constitutes a fish, examine the fish evolutionary tree, and discover intriguing examples that challenge traditional definitions. Investigate the remarkable size range of fish species and their significance in the animal kingdom. Gain insights into the distribution of water on our planet and understand the vast percentage of animal species represented by fish. This 23-minute lecture, presented by Thomas Evans, sets the stage for a deeper exploration of ichthyology, providing a solid foundation for further study in this captivating field of marine science.