士官，又称军士。一般国家的军队体制里，军人分为军官、士官及士兵三大体系，各有其任务执掌。大部分士官通常由士兵考试或选拔出来，不一定经过专业军校教育，负责辅佐军官、带兵和前线战术作为。

士官由低级逐阶晋升，通常分为下士、中士、上士，以及士官长（由低至高为：三等士官长、二等士官长、一等士官长）等军衔。编制上多以士官长担任士官督导长，士官担任班长，一个班下辖三到伍，每个伍为三人左右。编制的实际运用上，可以用较低军阶的人员担任高端职务，称为「占缺」。如以上士担任士官督导长。

在员额不足的部队中，会选拔资深或能力优异的士兵充当准下士（以兵代士），是为伍长。战时领导三至五人的步兵作战队伍。

A non-commissioned officer or noncommissioned officer (NCO, colloquially non-com) is a military officer who has not earned a commission. Such is also called sub-officer in some countries. Non-commissioned officers, in the English-speaking world, usually obtain their position of authority by promotion through the enlisted ranks. In contrast, commissioned officers hold higher ranks than NCOs, have more legal responsibilities, are paid more, and often have more non-military training such as a university diploma. Commissioned officers usually earn their commissions without having risen through the enlisted ranks. "mustang" is slang term in the United States Armed Forces, referring to a commissioned officer who began his or her career as an enlisted service member.