Every year almost 300 million scrap tires are generated in the United States. Recycled rubber obtained from scrap tires can be used in a number of beneficial ways. One of the most beneficial uses involves producing Ground Tire Rubber (GTR) from scrap tires and using the GTR to create Rubber-Modified Asphalt (RMA). RMA has been used in the U.S. since the 1960s, but extensive market adoption is yet to occur. Thus, a central question regarding RMA that still remains unanswered is, can RMA help eliminate scrap tire stockpiles in the U.S., boost pavement sustainability and longevity, and allow more miles of roads to be repaired? Researchers at the University of Missouri-Columbia tested this hypothesis in collaboration with the U.S. Tire Manufacturers Association (USTMA) and The Ray, a philanthropic organization dedicated to the discovery and implementation of sustainable transportation technologies. The resulting State of Knowledge (SOK) report provides an up-to-date review of RMA, including its historical development and use, production methods, field performance, economics, safety, driver comfort, environmental impact, and sustainability benefits. Knowledge gaps and recommendations for future research and investment are also assessed in the SOK report.