Entomology () is the scientific study of insects, a branch of zoology. In the past the term insect was less specific, and historically the definition of entomology would also include the study of animals in other arthropod groups, such as arachnids, myriapods, and crustaceans. This wider meaning may still be encountered in informal use.
Like several of the other fields that are categorized within zoology, entomology is a taxon-based category; any form of scientific study in which there is a focus on insect-related inquiries is, by definition, entomology. Entomology, therefore, overlaps with a cross-section of topics as diverse as molecular genetics, behavior, neuroscience, biomechanics, biochemistry, systematics, physiology, developmental biology, ecology, morphology, and paleontology.
Over 1.3 million insect species have been described, more than two-thirds of all known species. Some insect species date back to around 400 million years ago. They have many kinds of interactions with humans and other forms of life on Earth.
Entomology is rooted in nearly all human cultures from prehistoric times, primarily in the context of agriculture (especially biological control and beekeeping). The natural philosopher Pliny the Elder (23–79 CE) wrote a book on the kinds of insects, while the scientist of Kufa, Ibn al-A'rābī (760–845 CE) wrote a book on flies, Kitāb al-Dabāb (كتاب الذباب). However scientific study in the modern sense began only relatively recently, in the 16th century. Ulisse Aldrovandi's De Animalibus Insectis (Concerning Insect Animals) was published in 1602. Microscopist Jan Swammerdam published History of Insects, correctly describing the reproductive organs of insects and metamorphosis. In 1705, Maria Sibylla Merian published the book Metamorphosis Insectorum Surinamensium about the tropical insects of Dutch Surinam.
Early entomological works associated with the naming and classification of species followed the practice of maintaining cabinets of curiosity, predominantly in Europe.