The Classical Groups: Their Invariants and Representations is a mathematics book by Hermann Weyl (1939), which describes classical invariant theory in terms of representation theory. It is largely responsible for the revival of interest in invariant theory, which had been almost killed off by David Hilbert's solution of its main problems in the 1890s. Weyl (1939a) gave an informal talk about the topic of his book. There was a second edition in 1946. Source: Wikipedia (en)