The army rank of captain is a commissioned officer rank historically corresponding to the command of a company of soldiers. The rank is also used by some air forces and marine forces, but usually refers to a more senior officer. Today, a captain is typically either the commander or second-in-command of a company or artillery battery. In the Chinese People's Liberation Army, a captain may also command a company, or be the second-in-command of a battalion.

A company is a military unit, typically consisting of 100–250 soldiers and usually commanded by a major or a captain. Most companies are formed of three to seven platoons, although the exact number may vary by country, unit type, and structure.