Aco or Accault, Michel, fl. 1680-1702, French explorer. He became La Salle's lieutenant, being favored by that explorer because of his courage, prudence, and wide acquaintance with Native American languages. When La Salle reached the mouth of the Illinois River on his famous voyage down the Mississippi, he sent Aco with two companions to explore the upper reaches of the Mississippi. One of the companions was Father Louis Hennepin, who in his Nouvelle Decouverte made himself the hero of the expedition. Near the Falls of St. Anthony, which they were the first Europeans to see, the three were captured by members of the Sioux tribe and were released only through the energy and influence of Daniel Greysolon Duluth. Little is known of Aco's subsequent life except that he was a trader on the Illinois for many years and that in 1693 he married the daughter of a Kaskaskia chief. His name also appears as Ako.

ACO, AcO, or Aco may be:

• Automobile Club de l'Ouest, organizers of the famous 24 Hours of Le Mans auto race
• ACO (musician), J-Pop singer
• Alexander Christian Olsen, King of Fridley
• Allied Command Operations, the NATO strategic command
• Animal control officer
• Ant colony optimization algorithm
• Acetoxy group (AcO), a chemical functional group
• Air Cadet Organisation, collective name for the UK cadet forces sponsored by the Royal Air Force
• Aircraft Certification Offices, a body of the FAA.
• An abbreviation for American Composers Orchestra
• An abbreviation for Australian Chamber Orchestra
• Aco Records, a record company.