Paul Gordan (1837–1912), professor at the University of Erlangen, had given lectures on the theory of invariants, which were elaborated, reorganized, and completed by Georg Kerschensteiner who published them in two volumes. Gordan's lectures competed with the textbooks of A. Clebsch (1872), George Salmon (1859 and later) and Francesco Faa di Bruno (1881). It was Alfred Clebsch who had introduced Gordan to the invariant theory as early as 1860. Gordan was called “King of invariants” at his time. Gordan's lectures are republished in a textually unaltered reprint ... In the 19th century invariant-theory had been wide-spread, but the interest declined in the 20th century ... The reprint emphasizes how important invariant-theory once had been.