In this new edition of An Introduction to Pseudo Differential Operators, the style and scope of the original book are retained. A chapter on the interchange of order of differentiation and integration is added at the beginning to make the book more self-contained, and a chapter on weak solutions of pseudo-differential equations is added at the end to enhance the value of the book as a work on partial differential equations. Several chapters are provided with additional exercises. The bibliography is slightly expanded and an index is added.