This volume is the outgrowth of a series of lectures presented at a CBMS Regional Conference held at Texas Tech University in May 1972. In these lectures the author takes up several topics in the theory of linear partial differential equations, beginning with rather elementary, expository material, and going on to some of the current developments and techniques. The lectures are meant for the nonexpert, as an introduction to some of the current questions and ideas. Since the author wished to include some deep results, he has been technical on some occasions, but he has endeavored to describe the necessary background.