Unlike Army Reserve and Air Force Reserve members, National Guard members cannot be mobilized individually (except through voluntary transfers and Temporary Duty Assignments TDA), but only as part of their respective units.

However, since 11 September 2001 there have been a significant number of individual activations under Title 10 USC to support military operations (2001–Present); the legality of this policy has been a major issue within the National Guard.

For much of the final decades of the twentieth century, National Guard personnel typically served "One weekend a month, two weeks a year", with a portion working for the Guard in a full-time capacity as either Active Guard and Reserve (AGR), Army Reserve Technicians or Air Reserve Technicians (ART).

This changed dramatically during the 1990–91 Gulf War, and continued on to present day, with both the Federal Reserve Components and the National Guard increasingly utilized as an "operational" force for worldwide deployment.

More specifically, previous Department of Defense policy was that no National Guardsman would be involuntarily activated for a total of more than 24 months (cumulative) in one six-year period.