This dissertation chronicles the disciplinary development of ichthyology through the work of three prominent nineteenth-century naturalists: Georges Cuvier (1769-1832), Louis Agassiz (1807-1873), and Albert Günther (1830-1914). Cuvier argued that historical knowledge was necessary in order to understand the development of the natural sciences, and he devoted a significant portion of his career writing and lecturing on the development of these sciences. Cuvier’s zoological studies focused on vertebrates, and he wrote foundational works in comparative anatomy and paleontology. He applied techniques from comparative anatomy and paleontology to the study of fish. Toward the end of his career Cuvier began working on a catalog of fish with the assistance of Achille Valenciennes (1794-1865). Cuvier died before this work was finished and Valenciennes continued the 22-volume series. Günther’s work followed the methodologies promoted by Cuvier. Over the course of his career he produced a catalog of fish that described over 8000 species of fish. He also wrote the first English language textbook on ichthyology. Agassiz studied under Cuvier during the final six months of Cuvier’s life. Between the early-1830s and mid-1840s Agassiz studied fish fossils in the museums of Europe, describing over 1600 species of extinct fish. Agassiz’s study of fish paleontology influenced his interpretation of the fossil record and reinforced his belief that the geological evidence did not support evolutionary theory.