Peter Gustav Lejeune Dirichlet (1805-59) may be considered the father of modern number theory. He studied in Paris, coming under the influence of mathematicians like Fourier and Legendre, and then taught at Berlin and Goettingen universities, where he was the successor to Gauss. This book contains lectures on number theory given by Dirichlet in 1856-7. They include his famous proofs of the class number theorem for binary quadratic forms and the existence of an infinity of primes in every appropriate arithmetical progression. The material was first published in 1863 by Richard Dedekind (1831-1916), professor at Braunschweig, who had been a junior colleague of Dirichlet at Goettingen. The second edition appeared in 1871; this reissue is of the third, revised and expanded, edition of 1879; a fourth edition appeared as late as 1894. The appendices contain further work by both Dirichlet and Dedekind.