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Background The cause of amyotrophic lateral sclerosis (ALS) is unknown, but occupations have been explored as a potential proxy measure of risk. There is a substantial body of literature connecting military service to ALS. We aimed to summarize and assess the quality of this evidence. Methods Systematic review of the literature, including observational studies which explored one of the following exposures: general military service (army, air force, marines, or navy); or specific exposures associated with military service measured among military personnel. The outcome of interest was ALS incidence, which could include onset, diagnosis, or death from ALS. Results A total of 2642 articles were screened. Following exclusion, 19 articles remained for inclusion in the systematic review, including 1 meta-analysis and 18 original observational studies. Most studies were of moderate quality. In general, the relationship between military service was suggestive of an increased risk, particularly among Gulf War and WWII veterans. Exposure to pesticides (including Agent Orange) certain chemicals (exhaust, burning agents), heavy metals, and head trauma appeared to increase the risk of ALS among military personnel. Conclusions There is a possible association between military service and the subsequent development of ALS; however, the evidence was limited. Studies were generally hindered by small sample sizes and inadequate follow-up time. Future studies should endeavor to objectively measure specific exposures, or combinations thereof, associated with military service, as this will be of vital importance in implementing preventative strategies into military organizations.