From the reviews of the earlier editions: "An excellent second-reading text.  Should be accessible to any mathematician.  Highly recommended." - The Mathematical Gazette "An excellent text upon which to build and introduction to partial differential equations at the graduate level.  In addition...recommended for the nonspecialist." - American Mathematical Monthly Partial Differential Equations is a very well accepted introduction to the subject.  In it, the author identifies the significant aspects of the theory and explores them with a limited amount of machinery from mathematical analysis.  Now, in this fourth edition, the book has again been updated with an additional chapter on Lewy's example of a linear equation with solutions.