This textbook is a companion to previous books on welding metallurgy and weldability of stainless steels and Ni-based alloys published in 2005 and 2009, respectively. In retrospect, this book should have been published first because it lays the groundwork for many of the metallurgical and weldability concepts that are described in those texts. The subject matter in this book is based on a course that I have taught in the Welding Engineering Program at Ohio State University since 1986. The content is designed to provide engineers with the necessary background to understand the basic concepts of welding metallurgy and to interpret failures in welded components