This report presents some of the challenges that the U.S. Department of Defense (DoD) may face in fielding an artificial intelligence (AI) technology called reinforcement learning (RL) in DoD applications. RL has been credited with expanding the decisionmaking ability of machines beyond that of humans in playing complex games of strategy. The fact that RL-enabled systems can beat world experts in these games raises the question of whether such systems could outperform humans in DoD applications.