Even if in the last years several researches have studied the Friction Stir Welding (FSW) process, it should be observed that most of these studies are concerned with the butt joint and just a few of them extend to more complex geometries. It is worthy to notice that the acquired knowledge on FSW process of butt joints is not immediately extendable to lap and T-joints. The first observation is that in butt joints the surface to be welded is vertical, while in lap and T-joints it is horizontal and placed at the bottom of the top blank to be welded; in this way a major vertical component of the material flow is required to obtain sound joints. In the FSW of lap-joints four different geometrical configurations are possible—actually reducible to two—on the basis of the combination of the mutual position of the sheets to be welded and of the tool rotation direction, strongly affecting the process mechanics and the effectiveness of the final part. Furthermore, in the FSW of T-parts a proper clamping fixture is needed in order to fix the stringer during the process; such fixture is characterized by two radii, one for each side of the joints, corresponding to the radii between skin and stringer in the final welded part (corner fillets). Actually during the FSW process such radii must be filled by the flowing material. Consequently, an actual forging operation is required to force the sheet and the stringer material in fulfilling the radii of the clamping fixture, resulting in the radii of the T-joint. In other words, the material flow induced by the tool in the FSW process must be effective enough to get both the bonding of the two blanks and the fulfillment of the fixture radii. On the basis of the above observations, once the material of the blanks to be welded is selected, the most effective set of operating and geometrical parameters that optimize the FSW of butt joints will not, in all probability, work for the lap or T-joints. In particular, the tool geometries together with the tool feed rate and rotating speed must be redetermined in order to get an effective material flow and bonding conditions during the FSW process since the plastomechanics of the two processes are completely different. The specific peculiarities of the two processes must be properly investigated and the correlations between the characteristics of the materials to be welded and the mechanics of the welding configurations must be highlighted.