The automotive industry is facing mounting pressures in several areas including energy efficiency, net-zero emissions, safety, and affordability. Lightweighting is one of effective methods to address these challenges, since a 10% weight reduction can result in a 6-8% fuel efficiency gain. The global automotive lightweight materials market is also predicted to reach 148 billion USD by 2030. Vehicle lightweighting can be achieved via stronger materials such as advanced high-strength steels or lighter materials such as magnesium and aluminum alloys, along with the essential manufacturing methods. To maximize the lightweighting potential, an innovative “multi-material” concept has been adopted, where the characteristics of varied materials are optimized for the desired applications for lightweighting, cost effectiveness and value addition. The structural applications of multi-materials inevitably involve welding and joining. This poses significant challenges due to different thermophysical and mechanical properties and the related requirements on the safety, reliability and durability of welded joints. Multi-material welding between magnesium and other alloys represents a big challenge, since intermetallic compounds may occur to potentially cause premature failure. Some emerging solid-state joining techniques, such as ultrasonic spot welding, friction stir (spot) welding, have been developed to join the light alloys. In this talk, several examples on the joining of dissimilar magnesium-to-aluminum, magnesium-to-steel and aluminum-to-steel using ultrasonic spot welding will be presented. The weld interface experienced dynamic recrystallization during similar welding, while an intermetallic compound layer or eutectic layer was formed during dissimilar welding, depending on material combinations and welding parameters. To avoid the occurrence of intermetallic compounds, an interlayer (tin, zinc, etc.) was used during the multi-material welding. It was observed that the tensile lap shear strength and fatigue life of such welded joints were effectively improved. The evolution in the microstructure and texture as well as fatigue fracture mechanisms of the welded joints will also be presented.