William Stimpson was at the forefront of the American natural history community in the latter half of the nineteenth century. Stimpson displayed an early affinity for the sea and natural history, and after completing an apprenticeship with famed naturalist Louis Agassiz, he became one of the first professionally trained naturalists in the United States. In 1852, twenty-year-old Stimpson was appointed naturalist of the United States North Pacific Exploring Expedition, where he collected and classified hundreds of marine animals. Upon his return, he joined renowned naturalist Spencer F. Baird at the Smithsonian Institution to create its department of invertebrate zoology. He also founded and led the irreverent and fun-loving Megatherium Club, which included many notable naturalists. In 1865, Stimpson focused on turning the Chicago Academy of Sciences into one of the largest and most important museums in the country. Tragically, the museum was destroyed in the Great Chicago Fire of 1871, and Stimpson died of tuberculosis soon after, before he could restore his scientific legacy. This first-ever biography of William Stimpson situates his work in the context of his time. As one of few to collaborate with both Agassiz and Baird, Stimpson's life provides insight into the men who shaped a generation of naturalists—the last before intense specialization caused naturalists to give way to biologists. Historians of science and general readers interested in biographies, science, and history will enjoy this compelling biography.