What do experts envision as the largest technology-based challenges the U.S. Joint Force could confront by 2040? The authors develop and implement a three-stage approach to elicit and evaluate ideas from experts on technology challenges that are ?radically altering.? Twenty challenges were identified and evaluated by experts. These evaluations allowed the authors to classify those challenges into three clusters to help inform how the U.S. Department of Defense (DoD) could prioritize science and technology (S&T, ) investments.