Advanced industrial product development is faced with steadily growing demands for joining new materials, often in dissimilar combinations, implying complicated joining problems. It is therefore imperative that manufacturing of joints in high tech products should be treated with scientific engineering methods. This is, however, seldom done to an extent justified by its importance. Although traditional arc welding processes have been subjected to numerical analysis of isolated problems such as temperature analysis, weld pool dynamics, microstructural evolution as well as transient and residual stress/strain and distortion analysis the idea of modeling with the aim of optimizing the entire process is still in its infancy state, and when it comes to more sophisticated processes like resistance welding (RW) and friction stir welding (FSW) even more basic numerical studies are in an early stage of development. Both of these processes need complex thermo-mechanical calculations, microstructural predictions as well as thorough analysis of large plastic deformations in order to predict weld strength and optimum welding parameters as well as final geometry. These problems are challenging and of multi-physics nature involving complex mechanisms comprising several cross-disciplinary areas such as materials science, thermodynamics, solid and fluid mechanics as well as process technology and applied numerical analysis. The objective of the present project is to develop advanced numerical models applicable for simulation of RW and FSW aiming at possible optimization of the entire processes. Focus will be set on overall numerical modelling strategies specific for each of the two welding processes as well as more detailed investigations of microstructures, mechanical properties, strength and weld quality. The part of the project carried out at MAT is concernced with optimization of the welding processes using rational computational tools from optimal design and sensitivity analysis.