This is a textbook and reference work for ichthyology courses, which will also serve as a tool for ichthyologists, fisheries scientists, marine biologists, and vertebrate zoologists. It covers the basic anatomy and diversity of all 62 fish orders and focuses on the characteristic features of about 180 of the most commonly encountered fish families. Each family is diagnosed based on easily observable traits and clear photographs, many of which are in color and taken from live specimens. This guide will be distinguished by the use of photographs of preserved specimens, primarily from the Marine Vertebrate Collection of the Scripps Institution of Oceanography, complemented by X-rays and additional illustrations of important features. The aim is to support ichthyology students, fisheries scientists, marine biologists, vertebrate zoologists, and others who are interested in the subject or have an interest in it.