Stress in the military reality is often related to either mental health or performance. When it comes to mental health, most of the literature is concerned with the possible negative effects from exposure to combat-related trauma or other stressors related to deployment. Performance, on the other hand, is more often related to training. In this thesis, both areas are addressed in the following 5 studies. In Study I, stress related to deployment was measured before, during, and after deployment. The results showed that stress was lower during deployment compared to before or after. The study aims to highlight that all deployments are unique and not by nature inherently stressful. Study II continues to target deployment; the study looks at the relevance of assessing stress and mental health before and during deployment to predict postdeployment mental health. The results showed no predictive value over time but some correlation between pre- and during-measures. Still, we found that mental health screening is relatively easy to do and provides relevant data on current mental health status. Study III and Study IV are focused on military-specific training. In Study III, we looked at stress assessed through cortisol measures and cognitive performance during a conduct after capture training course. The aim was to evaluate the effectiveness of the scenario aimed at stressing the participants. Results showed that the training was effective and that stress levels were multiplied during the exercise. During training, the subjects showed difficulties recalling and utilizing strategies that they were taught due to the intense stress. However, there was no effect on cognitive performance when assessed directly after, indicating a short recovery time for cognition from after being exposed to the stressors. Study IV concerns “Breaching”; during breacher training, the operator is exposed to low-level blast (LLB). In this study, we looked closer at two outcomes: biomarkers of brain injury and cognitive performance. The biomarkers show a reactive response. In direct conjunction with the blasts, returning to baseline when followed up a few days later. There were no effects on cognitive performance due to the LLB exposure. Study V is an international collaboration with the Defense and Veterans Brain Injury Centre in the US. In that study, we used registry data from soldiers returning from Iraq or Afghanistan. They were screened for Traumatic Brain Injury, chronic pain, and other disorders. The objective was to see if the age at onset of the injury had any impact on the type and magnitude of symptoms. The results suggest that the younger soldiers with a still maturing brain are more susceptible to frontal lobe-related symptoms, while symptoms related to cognitive performance were slightly more noticeable in the older subjects. Overall, the thesis illustrates the importance of “measuring”, to gain a valid assessment both for stress management preserving mental health and performance as well. Mental Health in the military is mostly related to post-deployment assessment rather than proactive actions, bringing health closer to performance can increase awareness of the need for stress management strategies.