The transportation sector is the largest contributor to greenhouse gas emissions in the United States. One method being used to reduce greenhouse emissions related to the transportation sector is improving vehicle fuel efficiency through mass reduction. Reducing the mass of on-highway passenger vehicles by 10% can result in vehicle fuel economy improvements of as much as 6–8% if the powertrain is downsized to maintain equivalent performance. Some of the materials being investigated and implemented to reduce passenger vehicle mass include advanced high-strength steel, aluminum, magnesium, and polymer composites. Additionally, multimaterial structures that allow for optimal combinations of lightweight materials to achieve maximum weight reduction with lowest cost and best structural performance have recently become of particular interest. However, assembling multimaterial structures can be challenging due to differences in melting temperature and coefficient of thermal expansion of different materials, as well as formation of intermetallic compounds and galvanic corrosion potential. Joining technologies for lightweight multimaterial structures must address these challenges to be successful. This article highlights advances made in five different joining techniques: nondestructive evaluation of resistance spot-welded aluminum to steel, modeling of structural adhesives, temperature control of friction stir welds, ultrasonic welding of magnesium, and vapor foil actuation welding.