One of the best ways to reduce carbon emissions from transportation is to make vehicles lighter, whether they’re cars, trucks, planes, trains, or ships. In a typical light-duty car, every 10% reduction in weight increases fuel efficiency by 6-8%, with a 30% weight reduction allowing for efficiency improvements of up to 20-25%. The use of lightweight, strong materials like carbon-fiber reinforced plastics and metals such as aluminum, magnesium, and others present tremendous opportunities for vehicle weight reduction. However, these materials are currently too expensive to manufacture and cost-effective technologies for joining them together into lightweight structures don’t yet exist in many cases. To successfully enable dramatic reductions in greenhouse gas emissions from the transportation sector through vehicle lightweighting, we need to develop transformational new lightweight materials, inexpensive manufacturing processes, and new material-joining processes.