Friction Stir Welding (FSW) is a solid-state joining process that utilizes the heat produced between the material and a non-consumable rotating pin to join the desired materials or workpieces. This rotation causes a plasticized region of material to rotate about the tool. As the tool is moved through the material, the material on the leading edge enters the plasticized region and is swept around to the back of the tool where the lagging material is left to form a solid joint. Recently, there has been much development in the FSW of dissimilar materials. It has been successfully applied to weld magnesium, aluminum, steels, copper and other soft metals. The welding process for dissimilar metals is quite arduous and challenging because of the different melting temperature and thermal conductivity. Dissimilar welding technology has provided the opportunity to employ the advantages of both metals simultaneously based on the priority of local demands. A part of the growing need for dissimilar welding relates to joining aluminium and magnesium base metals. Individually, both metals are utilized extensively in the automotive and aerospace industries due to various advantages, respectively, including lightweight, high specific strength, and recyclability. Even so, some specific advantages may favor aluminium (such as higher strength or creep resistance) or magnesium (for higher damping capacity), and thus certain applications are in favor of one of the metals over the other. Like conventional welding methods, the intermetallic compound formation represents the main problem issue in FSW. A cause and effect diagram exposes many of the parameters that affect IMC formation and thickness. Heat cycle has a significant effect on IMC thickening phase transformation and the elimination of the amorphous phase. The hybrid techniques utilized in FSW for Al-to-steel joints reveal high joint strength in conjunction with high costs of setup and equipment. These techniques modify the steel flow around the rotated pin by adding an assisted heat source. This heat source should not significantly affect the microstructure of the Al alloys. The dissimilar joining of Cu to Al by FSW is still not widely employed because of low mechanical properties and formation of IMCs in large amount. Imperfections such as fragmental defects, voids, pores, and cracks are commonly found in dissimilar Cu-Al FSW system which is formed due to improper process parameters that consequentlyforms different IMCs and lead to the low mechanical properties. These IMCs also increases the hardness of joint area and that also makes the joint area brittle which is driving parameter for brittle fracture and low elongation. The present article provides a comprehensive insight into dissimilar materials joined by FSW technology. FSW parameters such as tool design, tool pin offset, rotational speed, welding speed, tool tilt angle and position of workpiece material in the fixture for dissimilar materials are summarizedin the present chapter. Additionally, welding defects, microstructure and intermetallic compound generation for dissimilar FSW have been also discussed in this article. Furthermore, the new developments and future scope of the dissimilar system have been addressed. © 2019 by Nova Science Publishers, Inc. All rights reserved.