Friction stir welding (FSW) and friction stir processing (FSP) are two solid state processes that involve heating by friction and plastic deformation. As a result of these inputs, mechanical properties differ from those of base materials. For application of these techniques, it is necessary to know the hardness, static, fracture and fatigue properties associated with materials welded or treated. This chapter summarizes studies on each technique for several alloys. Process parameters have a high influence on mechanical properties achieved and residual stress and distortion are also of high importance. Due to their nature, residual stresses are unavoidable and play a fundamental role on properties such as fatigue behaviour. Residual stress and its associated distortion in FSW and FSP are discussed. These techniques are applicable to a high number of different alloys and applications of multi-material joining applications. Combined with the effect of process parameters, it is virtually impossible to study the mechanical properties for each case, therefore property prediction is highly important. A summary of studies regarding this issue is also presented.