A non-commissioned officer (NCO) is a military officer who does not hold a commission. Non-commissioned officers usually earn their position of authority by promotion through the enlisted ranks. (Non-officers, which includes most or all enlisted personnel, are of lower rank than any officer.) 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).