Originally created in 1953, this was re-instituted in 1966 for Vietnam and 1991 for the Gulf War. It is awarded for any honourable active duty service in the periods 27 June 1950 and 27 July 1954, between 1 January 1961 and 14 August 1974, and between 2 August 1990 and 30 November 1995.

For the what counts as "active duty" the following are excluded:

Guard and Reserve forces personnel on short tours of duty to fulfil training obligations under an inactive duty training programme.;

any person on active duty for the sole purpose of undergoing a physical examination; and

any person on temporary active duty to serve on boards, courts, commissions and like organizations or on active duty for purposes other than extended active duty.

The National Defense Service Medal may be awarded to members of the Reserve Components who was ordered to Federal active duty, regardless of duration, except for the categories listed above. Any member of the Guard or Reserve who, after 31 December 1960, became eligible for the Armed Forces Expeditionary Medal, Vietnam Service Medal or the Southwest Asia Service Medal was also eligible for the National Defense Service Medal.