A U.S.-Japan expert workshop on mobile alert and warning was held online 8-10 September 2021.Fun-ded by the Japan Foundation's Center for Global Partner-ship(CGP)and responding to the Sendai Framework for Disaster Risk Reduction 2015-2030,the workshop com-pared U.S.and Japanese mobile alert and warning contexts,systems,policies,and messages to investigate possibilities for international harmonization of mobile device-based early warning.The workshop's sessions revealed two interrelated issues that repeatedly surfaced among work-shop participants:culture and policy.The workshop illu-minated several possibilities and problems confronting U.S.,Japanese,and global stakeholders as they develop,deploy,and seek to improve the effectiveness of mobile alert and warning systems and messages.