The purpose of this study is to evaluate the cognitive and neurophysiological effects of chronic exposure to repeated low-level blast overpressure. Previous studies show converging evidence for a neurophysiological effect from cumulative exposure to blast, which is consistent with subjective reports of cognitive impairment by individuals trained to use explosives to gain entry to structures (breachers). This study will expand on these findings by examining a larger cohort of experienced breachers who may be incurring a cumulative effect of low-level blast exposure over the course of several years. We will evaluate up to 20 participants from each of three groups: breachers, artillery control personnel, and unexposed individuals. Additionally, we will ask each of the above to bring a close companion for a set of questionnaires and an interview to capture changes in functioning that participants may not be able to self-assess. Participants will travel to the NIH for evaluation of cognitive and neurophysiological measures. The procedures include neuropsychological testing, blood analysis for biomarkers, vestibular and auditory testing, and neuroimaging studies using diffusion tensor imaging, susceptibility weighted imaging, perfusion imaging, imaging with Gadolinium contrast, functional magnetic resonance imaging, and, where there is evidence of a sleep disorder, polysomnography. Participants will also be invited back for a 1-year, follow-up assessment to look for progression of any changes or differences from the control groups. This study will be conducted in collaboration with the Naval Medical Research Center (NMRC) and the Walter Reed Army Institute of Research (WRAIR) under a Research Collaborative Agreement (RCA). The complementary DoD protocol will be reviewed by the DoD IRB and includes this NIH protocol. The complementary DoD protocol includes data collection at field settings, activity NIH is not engaged in and not described in this protocol.