The title or rank of Captain was widely used in military organizations and some civilian contexts, and typically indicated an individual with leadership responsibilities.
In ground-based military organizations, Captains were typically company commanders, and almost always ranked immediately above Lieutenants. Captains led companies in both the Grand Army during the Clone Wars and in the Imperial Army during the Galactic Civil War, but in the Imperial forces, they could also serve as support officers, adjutants and aides-de-camp at battalion level or above.
Most starfighter corps followed army-style systems in their rank structure, but this did not always translate into giving officers of the same rank similar responsibilities. In the Imperial military and some early New Republic formations, a Captain typically led a Squadron of twelve starfighters, but by the outbreak of the Yuuzhan Vong invasion, a Captain in New Republic Starfighter Command could often command a mere Flight of four fighters.
In naval terminology, "Captain" could be a specific rank, but also a title held irrespective of rank. Any military officer with a specific commission to command a capital ship might be styled as Captain.
Within the chain of command, a specific rank of Captain was found in most naval forces, although the exact significance of the rank varied. In many naval services, the rank of Captain indicated a command-grade officer analogous to an army Colonel, above Commander but below Commodore; this was the situation in the New Republic, Galactic Alliance, and Chiss Expansionary Defence Force, and probably in the Old Republic and Rebel Alliance before them. Typically, officers holding the rank of Captain would hold the command of a single warship, but some Captains were responsible for entire squadrons of capital ships, and not all warships were commanded by officers holding the substantive rank of Captain.
For example, in the last years of the Old Republic, Lorth Needa commanded the Carrack-class light cruiser, RSS Integrity as a Lieutenant Commander, while the six Dreadnaught Cruisers of Outbound Flight were each under a Commander, with a single Captain in overall command. Similarly, it seems that the Captain in command of the Rendili Home Defense Fleet's flagship was a senior officer with extensive command authority. Occasionally, a Captain could serve in a subordinate role: for instance, Captain Theb Tobbra was first officer of the New Republic heavy crusier Indomitable during the Koornacht Crisis, with Commodore Brand serving as the ship's captain, although Brand also acted as at least de facto commander of a larger squadron.
In the Imperial Navy, the situation was more complex. In terms of the levels of the command hierarchy, the rank of Captain of the Line seems to have been broadly equivalent to the rank of Captain in other navies. An officer with this rank might lead a tactical group of corvettes, frigates or cruisers, command a large ship such as a Star Destroyer, or else serve as an Admiral's adjutant. There is also some evidence for lower grades of naval Captain in the Imperial service, who might command capital ships as large as Dreadnaught Cruisers, but whose position in the rank hierarchy broadly corresponded to that of a Lieutenant-Commander or Commander in the fleets of the Old Republic, Rebel Alliance and New Republic.
There was a great deal of prestige attached to the post of ship's captain in the Imperial fleet, and the ethos of the Navy honoured men who prized and cultivated their relationship with a specific ship: captains might often refuse promotions to administrative posts, or even turn down the command of larger and more capable warships.
Civilians in command of a private vessel, even a light freighter such as the Millennium Falcon, were also addressed as "Captain".
From the early days of the Galactic Republic, the Bureau of Ships and Services, or "BoSS", was responsible for the licensing, certification, and registration of starship captains. In order to be licensed, a civilian captain during the Imperial era needed to pass numerous oral, written, and practical tests. They also needed a standard background check, documentation of at least ten years as a spacer, and a 300 credit fee. The fee was increased to 500 credits to expedite licensing, which eliminated much of the testing and flight time requirements. Many smugglers operated with forged documents, however, which cost thousands of credits but avoided other legal entanglements.
Captain's licenses were required to legally buy a starship, obtain an arms load-out permit for a starship, or obtain copies of the Spacer's Information Manual from the Imperial Space Ministry. Operating a starship without a captain's license was a Class Four Infraction under the Imperial Penal Code.