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In 2007, MIL-STD-461 celebrated its 40th birthday. In 2007, the DOD released MIL-STD-461F, the latest version of the fundamental military Electromagnetic Compatibility (EMC) specification. This paper discusses Electromagnetic Interference (EMI) specifications prior to MIL-STD-461 and will outline the evolution and changes that MIL-STD-461 has undergone over its first 40 years of existence. MIL-STD-461 is applied to all active Military electronics installed in DOD platforms. MIL-STD-461 is a set of requirements intended to serve a wide range of platforms from trucks to ships to aircraft to fixed installations, and many different applications (e.g., above deck and below deck on a Navy ship). MIL-STD-461 is used as is by many countries around the world and has been adapted and is the basis for many other EMI specifications by countries throughout the world. The US military first encountered Radio Frequency Interference (RFI), the acronym precursor to Electromagnetic Interference (EMI), some time prior to World War I when a radio was first installed on a vehicle. However little is known about early efforts to address RFI. At that point the field of Electromagnetic Interference was created. From that point to the creation of MIL-STD-461 the field of EMI was in its infancy. There was a proliferation of Interference and Susceptibility specifications by the different services. In 1960, the US Department of Defense (DoD) created a comprehensive Defense Radio Frequency Compatibility Program (later renamed Electromagnetic Compa-tibility Program) that focused the Military Services R&D programs "to provide a means whereby electromagnetic compatibility should be 'built into' military communications-electronics equipment in the research and development stage". In 1967 MIL-STD-461 was created. With its sister documents MIL-STD-462 and MIL-STD-463 it was intended as a tri-service EMI specification. MIL-STD-461 delineated the EMI requirements. MIL- STD-462 detailed - the prescribed measurement methodology and definitions and acronyms were the basis of MIL-STD-463. From then until now MIL-STD-461 has gone through revisions A through F and many different notices. The changes have been an attempt to make an EMI specification that truly represents the existing electromagnetic environment and to make a test specification that ensures that the test data is reliable and repeatable at the many different test laboratories around the country. The intention of MIL-STD-461 is that it serves as a box test that is a gateway to installing equipment on the intended platform. If a unit passes MIL-STD-461 there is confidence that the unit will operate in its intended electromagnetic environment on its intended platform. The ultimate EMI requirement is error free operation of the equipment in its intended electromagnetic environment but MIL-STD-461 stands as the best vehicle to get to that point.