This book introduces and reviews the academic papers exchanged at the 2021 Annual Assembly of the International Institute of Welding (IIW), covering research progress of the international welding community in various fields, including additive manufacturing, thermal cutting and thermal spraying, arc welding and filler metals, pressure welding, high-energy beam processing, non-destructive testing and quality assurance of welded structures, micro and nano connections, welding health, safety, and environment, metallic weldability, welded joint performance and fracture prevention, pressure vessels, boilers and pipes, arc welding processes and production systems, fatigue of welded components and structures, welding education and training, welding structure design, analysis, and manufacturing, polymer connections and adhesive technology, soldering and brazing and diffusion welding technology, welding physics, and welding training and certification. In the meantime, it introduces the overall process and comprehensive activities of the 2021 Annual Assembly of IIW. This book is a reference for technicians and managers engaged in welding, research in related fields, engineering applications, certification and training, and society building. It also suits senior graduate students studying welding and related disciplines.