It is known that stress and trauma can cause psychological complaints, and that these psychological complaints are accompanied by abnormal activity in certain brain areas. Influencing this brain activity could promote the prevention and treatment of psychological complaints, particularly among military personnel and veterans. For them, current therapies are often insufficiently effective. Brain activity can be influenced in a safe way with non-invasive brain stimulation. The research in this dissertation focuses on a specific brain stimulation technique called 'tDCS', an abbreviation of transcranial direct current stimulation. Previous research on brain stimulation with tDCS in healthy volunteers shows that tDCS can affect emotional and cognitive processes in a controlled test room environment. This dissertation translated that research into effects on stress-related mental health in military personnel and veterans. The study results in this dissertation do not provide convincing evidence for positive effects of tDCS on emotional control or recovery from symptoms in military personnel and veterans. That is, we found no relevant effects of the specific way tDCS was applied in two placebo-controlled intervention studies among military personnel and veterans with anxiety, aggression or post-traumatic stress symptoms, and among military personnel without mental health symptoms. Our findings suggest that brain stimulation with tDCS is not yet ready for use in the context of mental health in military personnel and veterans, and underscore the importance of further research into the mechanism of action of tDCS and effective applications.