A senior enlisted advisor (SEA) in the United States Armed Forces is the most senior enlisted service member in a unit, and acts as an advisor to the commanding officer. Formally, E-9 billets for the senior enlisted advisor are established at service unit, command, major command, force, or fleet levels to the SEAs/CSELs of DoD Agencies and the Senior Enlisted Advisor to the Chairman of the Joint Chiefs of Staff. SEAs are also known as command senior enlisted leaders (CSEL). Always a non-commissioned officer, the SEA is the main link between the commanding officer and the enlisted service members under his or her charge.

The commanding officer (CO) or commander, or sometimes, if the incumbent is a general officer, commanding general (CG) or general officer commanding (GOC), is the officer in command of a military unit. The commanding officer has ultimate authority over the unit, and is usually given wide latitude to run the unit as they see fit, within the bounds of military law. In this respect, commanding officers have significant responsibilities, duties, and powers.