Origins
Military lore has it that a wealthy American World War I flying ace had squadron medallions struck for his comrades to be carried at all times.
The Challenge
The flyer was captured by Germans, and after his escape in civilian clothes he produced the coin as identification to French soldiers who accepted it and gave him safe passage.
Elite Membership
Challenge coins were particularly popular during World War II and the Korean War and were often informally awarded to military personnel upon graduation from training exercises.
The Look
Coins can range from the simple to ornate and usually feature the military unit's logo or insignia, identifying number and motto.
Non-Military
The military challenge coin has spawned a cottage industry of similar coins adopted by police and fire departments and civilian clubs and fraternities.
Presidents' Collections
Former President Bill Clinton has a collection of challenge coins given to him from military personnel and former President George W. Bush received one from a Marine combat unit on a 2007 visit to Iraq.