The United States has provided benefits in varying degrees to those who have worn the uniform and suffered disabilities in service to the nation. In general, a veteran is entitled to compensation for disabilities incurred in or aggravated during active military, naval, or air service. It should be noted that not all persons who served in the military are considered veterans for purposes of veterans benefits. Veterans could meet the burden of proving that their disabilities are service-connected through their military records, which may clearly describe and document the circumstances and medical treatment for an injury or an illness incurred while in service as well as any resulting disability. However, where the manifestation of the disability is remote from the veteran's service and any relationship between the disability and service is not readily apparent, the burden of proving service connection can be a challenge. In such circumstances, Congress and the Department of Veterans Affairs (VA) have relied on presumptions. In the context of VA claims adjudication, a presumption could be seen as a procedure to relieve veterans of the burden to prove that a disability or illness was caused by a specific exposure that occurred during service in the Armed Forces. When a disease is designated as presumptively service-connected, the individual veteran does not need to prove that the disease was incurred during service. The legislative history of veterans' disease presumptions dates back to 1921 when Congress established a presumption of service connection with an amendment (P.L. 67-47) to the War Risk Insurance Act (P.L. 63-193). It established presumptions of service connection for tuberculosis and neuropsychiatric disease (known today as psychosis) occurring within two years of separation from active duty military service. In the following years, additions to the presumptive list were made by regulation, executive order, and legislation. In the past 22 years, Congress has on three separate occasions created presumptive programs for three distinct groups of veterans: the so-called atomic veterans, who were exposed to radiation from above-ground nuclear tests and the atomic bombs detonated in Japan; Vietnam veterans; and Gulf War veterans. In addition, Congress has added certain disease conditions to the list of presumptions for specific groups of veterans such as former prisoners of war (POWs). In 1991, the Agent Orange Act (P.L. 102-4) established for Vietnam veterans a presumption of a service connection for diseases associated with exposure to Agent Orange and other herbicides. For the first time, this act required the VA to contract with the Institute of Medicine (IOM) to conduct, every two years, a scientific review of the evidence linking certain medical conditions to herbicide exposure. The VA was instructed to use the IOM's findings, and other evidence, to issue regulations establishing a presumption for any disease for which there is scientific evidence of an association with herbicide exposure.