This book presents a modern viewpoint of welding thermomechanics and provides a unified and systematic continuum approach for engineers and applied physicists working on the modelling of welding processes. The theory presented includes developments in the areas of welding thermodynamics, thermoplasticity and numerical methods. The book describes the calculation of thermal stresses in welding structures, the theory of heat conduction in welding, and the basic equations of thermoplasticity, then applies the concepts to solutions of thermoplasticity problems and to thermal stresses in arc welding, laser welding, spot welding, electroslag welding and friction welding.