private coeducational university and research institute in Pasadena, California, U.S., emphasizing graduate and undergraduate instruction and research in pure and applied science and engineering. The institute comprises six divisions: biology; chemistry and chemical engineering; engineering and applied science; geologic and planetary sciences; humanities and social sciences; and physics, mathematics, and astronomy. Total enrollment is approximately 2,000, of which more than half are graduate students.

Superbly equipped, and staffed by a faculty of some 1,000 distinguished and creative scientists, Caltech is considered one of the world's major research centres. Dozens of eminent scientists (including many Nobel Prize winners) have worked and taught there, including physicists Robert Andrews Millikan (Millikan, Robert Andrews), Richard P. Feynman (Feynman, Richard P.), and Murray Gell-Mann (Gell-Mann, Murray); astronomer George Ellery Hale (Hale, George Ellery); and chemist Linus Pauling (Pauling, Linus). In 1958 the Jet Propulsion Laboratory at Caltech, operating in conjunction with the National Aeronautics and Space Administration, launched Explorer I, the first U.S. satellite, and it subsequently conducted other programs of space and lunar exploration. Caltech operates astronomical observatories at Owens Valley, Mount Palomar (Palomar Observatory), and Big Bear Lake in California and at Mauna Kea (Mauna Kea Observatory) in Hawaii. Other institute facilities include a seismological laboratory in Pasadena and a marine biological laboratory at Corona del Mar.

Caltech was established in 1891 as a school for arts and crafts. First called Throop University and later Throop Polytechnic Institute, it assumed its present name in 1920. The institute originally included curricula in business and education, but in 1907 it dropped several programs and began specializing in science and technology, with a focus on creativity and research.