This book has evolved from a two-term graduate course in partial differential equations which I have taught at Northeastern University many times since 1980. The first term is intended to give the student a basic and classical introduction to the subject, including first-order equations by the method of characteristics and linear second-order equations which arise in mathematical physics: the wave equation, Laplace equation, and heat equation. All of this material is more than adequately covered by many textbooks which are readily available. The second term, however, is intended to introduce the student to a wide variety of more modern methods, especially the use of functional analysis, which has characterized much of the recent development of partial differential equations. This latter material is not as readily available, except in a number of specialized reference books. This textbook is intended to bridge this gap by providing the student with a basic introduction to the subject and an exposure to some of the more modern methods.