This is a corrected reprint of Lang's well-known textbook. It covers all of the basic material of classical algebraic and analytic number theory, giving the student the background necessary for the study of modern algebraic number theory. Part I introduces some of the basic ideas of the theory: number fields, ideal classes, ideles and adeles, and zeta functions. Part II covers class field theory and Part III is devoted to analytic methods, including an exposition of Tate's thesis, the Brauer-Siegel theorem, and Weil's explicit formulas.