Brain stimulation research has moved far and fast. Different forms of neurostimulation in humans have now been shown to boost our ability to learn and perform motor actions, to pay attention to events in the environment, to recall information in memory, and to exercise self-control. [Brain stimulation] techniques and their applications hold great promise for understanding the machinery of human mental processes, with hope for treating patients suffering from brain injury and disease. Except that ... anything that can boost or rehabilitate human abilities could also be exploited for military or security purposes, as well as by questionable private enterprises. Predictably, some of the major innovations in brain stimulation research are being funded by the US military. What army wouldn't take advantage of a method that could make soldiers more alert, faster to react, faster to learn, less likely to binge-drink off duty, and more compliant with authority? What intelligence agency wouldn't embrace a technology that could help their operatives become better liars, or which limits the ability of prisoners to lie under interrogation? Applications like these, in turn, raise ethical questions. Would a soldier be able to refuse brain stimulation, and if not, wouldn't that violate the principle of consent underlying 'medical' interventions? Going deeper we might ask whether it is ethical for neuroscientists and psychologists to collaborate with organisations whose ultimate research objective is to develop more efficient ways of killing people.