This paper addresses candidate technologies for attaching steels to selected lightweight materials. Materials of interest here include aluminum and titanium alloys. Metallurgical challenges for the aluminum-to-steel and titaniumto- steel combinations are first described, as well as paths to overcome these challenges. Specific joining approaches incorporating these paths are then outlined with examples for specific processes. For aluminum-to-steel joining, inertia, linear, and friction stir welding are investigated. Key elements of success included rapid thermal cycles and an appropriate topography on the steel surface. For titanium-to-steel joining, successful approaches incorporated thin refractory metal interlayers that prevented intimate contact of the parent metal species. Specific welding methods employed included resistance mash seam and upset welding. In both cases, the process provided both heat for joining and a relatively simple strain path that allowed significant bond line forging without rupture of the separating interlayer.