A non-commissioned officer (NCO) is a military officer who does not hold a commission. They usually earn their position of authority by promotion through the enlisted ranks. In contrast, commissioned officers usually enter directly from a military academy, officer training corps (OTC) or reserve officer training corps (ROTC), or officer candidate school (OCS) or officer training school (OTS), after receiving a post-secondary degree.

The NCO corps usually includes many grades of enlisted, corporal and sergeant; in some countries, warrant officers also carry out the duties of NCOs. The naval equivalent includes some or all grades of petty officer. There are different classes of non-commissioned officers, including junior (lower ranked) non-commissioned officers (JNCO) and senior/staff (higher ranked) non-commissioned officers (SNCO).

All non-commissioned officers are considered junior to any commissioned (or warrant) officer, although there are rare instances where an NCO may hold a varying level of authority over a commissioned officer. The non-commissioned officer corps has been referred to as "the backbone" of the armed services, as they are the primary and most visible leaders for most military personnel. Additionally, they are the leaders primarily responsible for executing a military organizations mission and for training military personnel so they are prepared to execute their missions.

NCOs are charged with the care, training, education, and readiness of every soldier in the U.S. Army. They provide inspiration, purpose, motivation, direction, and discipline to the troops. NCOs are responsible for executing a military organizations mission and for training military personnel so they are prepared to execute their missions. They are leaders of soldiers and are responsible for the welfare of their soldiers.