Military personnel are required to perform effectively in extreme enviroments. Optimal performance in combat enviroments is a complex process and its neural basis is poorly understood. Understanding the factors that influence how an individual is able to perform to a high standard and cope with the demands of the situation while under extreme operational stress is vital. As stressful events can have a lasting impact on soldiers and while for some deployment can lead to positive change for others it can increase the risk of suffering from post-traumatic stress disorder (PTSD). To better understand how soldiers are able to perform effectively, in the first study of the thesis we developed a psychometrically robust measure of mental robustness that was informant rated and relevant to combat operations. The measure assesses a soldier’s ability to make decisions under pressure and their ability to function effectively when faced with emotional challenging situations as two separate dimensions. A second study confirmed the factor structure of the measure and also provided initial evidence for its construct validity. The measure underpinned our final study (Study 3) which combined psychometric measures, behavioural and functional imaging to produce a deeper understanding of the relationship between activity in key brain regions and key components of robustness. Study 3 assessed soldier’s ability to make decisions under pressure when presented with combat relevant stimulus. The study employed two tasks; Task 1 required individuals to attend to emotional aspects of the stimuli as they would do in during combat and Task 2 required soldiers to attend to the non-emotional aspects of the stimuli. Our findings suggest that robustness acts as a resistance resource and although it does not protect against PTSS it does allow a curvilinear relationship between PTSS and performance. The ultimate goal of this thesis is to better understand the critical factors required for optimal military performance during deployment. This will allow more targeted training that will help highly motivated individuals achieve excellence.