This self-contained and formal exposition of the theory and applications of pseudo-differential operators is addressed not only to specialists and graduate students but to advanced undergraduates as well. The only prerequisite is a solid background in calculus, with all further preparation for the study of the subject provided by the book's first chapter. This chapter introduces the fundamental concepts of spaces of functions and Fourier transforms, and covers such topics as linear operators, linear functionals, dual spaces, Hilbert spaces, distributions, and oscillatory integrals. The second chapter develops the theory of pseudo-differential operators themselves on the basis of elementary calculus and concepts presented in the opening chapter, while the third chapter extends the theory of Sobolev spaces. The major applications of the theory, most of them the result of work done since 1965, are in the study and solution of linear partial differential equations, which are found in many branches of pure and applied mathematics and are ubiquitous throughout the sciences and technology. The final seven chapters of Pseudo-Differential Operators take up a range of applications, and deal with such problems as hypoellipticity, local solvability, local uniqueness, index theory, elliptic boundary values, complex powers, initial values, well-posedness, the fixed point theorem of Atiyah-Bott-Lefschetz, Fourier integral operators, and propagation of singularities. For this English edition, the last chapter has been greatly extended and appendixes added in order to present the latest developments of the subject. Multiphase Fourier integral operators are applied to initial-value problems, the micro-local theory is developed from the notion of the "wave front set," and the Nirenberg-Treves existence theorem for the solutions of partial differential equations is discussed. The systematic use of the "multiple symbols" introduced by K. O. Friedrichs provides elegant proofs of otherwise lengthy developments.