In military organizations, the commanding officer (CO) is the officer in charge of a military unit. Typically the commanding officer has ultimate authority over the unit, and is usually given wide latitude, within the bounds of law. In this respect, Commanding Officers have significant responsibilities (for example, the use of force, finances, Geneva Conventions), duties (to higher authority, mission effectiveness, duty of care to personnel) and powers (for example, punishment of personnel to certain limits of military law.)

Commanding officers may be of any rank of officer; usually there are more officers than command positions available, and time spent in command is generally a key aspect of promotion, so the role of commanding officer is highly valued, and in theory only goes to the best officers.

The commanding officer is often assisted by an Executive Officer (XO) or Second-in-Command (2i/c), who handles personnel and day-to-day matters. Larger units will have a whole collection of staff officers .

In the British Army and Royal Marines, the title of Commanding Officer is reserved for commanders of 'major units ' (regiments, battalions and similar sized units), almost invariably holding the rank of lieutenant colonel and usually referred to within the unit simply as "the Colonel". The commander of a 'minor unit ' (company, squadron or battery) is referred to as the Officer Commanding (OC). Officers and NCOs in charge of platoons, troops and sections are just referred to as commanders (platoon commander, troop commander, section commander). In the Royal Air Force, the title of CO is reserved for the commanders of squadrons and wings.

In the Royal Navy and United States Navy, Commanding Officer is the official title of the commander of any ship, unit or installation. However, if at the rank of Captain or below, the Commanding Officer is referred to as "the Captain" no matter what his actual rank, or informally as "Skipper."

See: List of military commanders