The Army & Air Force Exchange Service (AAFES, also referred to as The Exchange and post exchange/PX or base exchange/BX) provides goods and services at U.S. Army, Air Force, and Space Force installations worldwide, operating department stores, convenience stores, restaurants, military clothing stores, theaters and more nationwide and in more than 30 countries and four U.S. territories. The Exchange is headquartered in Dallas, Texas, and its director/chief executive officer is Tom Shull. The largest of the United States Department of Defense's exchange services, it is No. 51 on the National Retail Federation's Top 100 Retailers list.

In addition to their retail support for the military, the Exchange outfits troops with combat uniforms at-cost, and serves approximately 3.4 million school lunches per year for children attending Department of Defense Dependents Schools overseas.

As of Veterans Day, 11 November 2017, military exchanges started offering online exchange shopping privileges to an estimated 18 million honorably discharged veterans. Disabled veterans, Purple Heart recipients and certain caregivers were given in-store shopping privileges in 2020. DoW and Coast Guard employees can shop in stores and online as of 1 May 2021.

For more than 100 years before the post exchange system was created, traveling merchants known as sutlers provided American soldiers with goods and services during times of war. Sutlers served troops at Army camps as far back as the French and Indian and Revolutionary wars.

Complaints of sutlers charging premiums for substandard merchandise were widespread during the Civil War, and in 1867, the War Department created the post trader system. While intended to prevent the unscrupulous practices of sutlers, the post trader system still subjected troops to over-inflated prices and was rife with bribery and corruption.

On 29 November 1880, Col. Henry A. Morrow, seeking to quell disciplinary problems resulting from troops visiting disreputable places of amusement in nearby towns, established the first American military canteen at Vancouver Barracks. There, troops were provided newspapers and magazines, played billiards and cards, and could obtain light food and drink without leaving post.

The idea was so successful that other posts began establishing canteens across the frontier, providing troops with not only a place to socialize but obtain daily necessities at affordable prices. In 1889, the War Department issued General Orders No. 10, authorizing commanding generals to establish canteens at army posts. Like the modern-day exchange system, these canteens were largely financially self-sustaining.

In February 1892, the secretary of war ordered that canteens be henceforth referred to as "post exchanges." This change was due to the popular association of the word "canteen" with the bawdy, immoral behavior alleged to occur in the canteens of foreign armies. By 1895, post traders had been almost entirely replaced on Army posts by post exchanges.