This book describes the weldability aspects of many structural materials used in a wide variety of engineering structures, including steels, stainless steels, Ni-base alloys, and Al-base alloys. The basic mechanisms of weldability are described and methods to improve weldability are described. Specific topics include solidification and liquation cracking, solid-state cracking, hydrogen cracking, fracture and fatigue, and corrosion. Methods for interpretation of weld failures using computational and characterization techniques are described.