Thaddeus William Harris first made his living as a physician and for many years thereafter as Harvard librarian. He also taught natural history in Harvard College—Henry David Thoreau was one of his students—but his desire for a full-time professorship was never realized. He is chiefly remembered as a naturalist and is generally considered the 'founder of applied entomology' in the United States. His historical reputation is linked to his Treatise on Some of the Insects Injurious to Vegetation. Going beyond the Treatise and examining Harris's life through his correspondence, reveals a picture that is more complex than his traditional reputation would suggest. In addition to a review of his familial and scientific origins, the author explores how Harris tried to build a scientific career, and looks at his work as an academic librarian. His research and writing per se is examined. While his work on insects as agricultural pests is well-known, in the 1830s Harris prepared the earliest systematic listing and classification of American insects. Most importantly, in his more specialized studies, he became interested in nocturnal Lepidoptera (moths), a group not much studied in America at the time. Here, Harris brought to bear his great knowledge of life histories of insects that was so germane to his agricultural effort, as well as some innovative uses of wing vein patterns, as aids to taxonomy. The book discusses his publishing strategies for scientific and popular work and his relations to individuals and organizations in the scientific community. Harris's well-formulated views on correct personal and communal conduct in natural history presents the context for a consideration of scientific practice in his era. The study also delves into his political and religious beliefs and his attitudes to the natural world and how these related to his scientific program.