Friction stir welding is a solid-state process that could be beneficially used for joining and repair of light metal alloys in transportation and defense applications. In this study, various applications, processes, and resulting properties of friction stir welds have been explored. First, the effects of various processing parameters on the resulting weld microstructures were studied. Second, tensile properties and fatigue crack growth mechanisms in friction stir welded 6061-T6 alloys were investigated. Fatigue crack propagation responses of the base and friction stir processed materials were studied in ambient conditions using compact tension specimens and multiple stress ratios, R=0.1, 0.5, and 0.7. Third, various exploratory studies were conducted to determine the feasibility of novel friction stir welding techniques for joining of dissimilar materials, porosity reduction, creation of in-situ metal-matrix composites for local reinforcement, and welding of die casting alloys. These findings will be systematically presented and discussed.