In many navies they are known as 'One Star' officers. (Admirals rank upwards with more stars).
Commodore derives from the French commandeur, which was one of the highest ranks in orders of knights. The Royal Netherlands Navy also used the rank of commandeur from the end of the 16th century and it was used for a variety of temporary positions until it became a conventional permanent rank in 1955. The Royal Netherlands Air Force has adopted the English spelling of commodore for an equivalent rank.
Use of the term "commodore" in the Royal Navy dates to the mid-17th century: it was first used in the time of William III. There was a need for officers to command squadrons, but it was not deemed desirable to create new admirals. Captains assigned squadron command were given this title, but it was not an actual rank. The officer so designated kept his place on the list of captains. In 1748 it was established that captains serving as commodores were equal to brigadier generals.
The Royal Navy commodore eventually became split into two classes. Those of the first class had a captain under them to command their ship. Those of the second class commanded their own ship as well as the squadron. In 1783, commodores of the first class were allowed to wear the uniform of a rear admiral, a distinction which continued with some variation until the two classes of commodore were consolidated in 1958. In 1996 the rank of commodore was made a substantive rank in the Royal Navy: previously it had been merely a temporary rank, and commodores used to revert to the rank of captain at the end of their posting (and captains could be promoted directly to Rear Admiral).
Commodores first class, while wearing the sleeve stripes of a rear admiral, had gold lace-covered epaulettes and shoulder-boards with a crown, two stars and anchor (also worn by other commodores but only with formal uniforms). They flew a swallow-tailed pennant with the St George's cross, but without the disc that appeared on the pennant of a second class commodore. Commodores second class wore a single 1.75 in. wide row of lace below a ring measuring 1.75 in. in diameter on both sleeve and shoulder-board (and this is the insignia worn by modern commodores). Their uniforms are otherwise the same as for captains.
Some Commonwealth countries have replaced Commodore with an equivalent flag rank, the (correct) insignia of which is a single 1.75 in. wide row of sleeve lace below a gold lace ring with a diamater of 2 in., and a crown (or comparable emblem for republics) with a crossed sword and baton on a gold lace-covered shoulder-board. (There is some variation due to misconceptions about the status of stars in Commonwealth-style rank insignia.) The rest of the uniform is identical to that of a rear admiral.
Commodores of the United States Navy have had a more complicated history. Congress was unwilling to authorize any admirals in its service until 1862, so considerable importance was attached to the office of commodore. Like its Royal Navy counterpart, the American commodore was not a higher rank, but a temporary assignment for captains. As Herman Melville wrote in White Jacket, 1849,
- An American commodore, like an English commodore or a French chef d'escadre, is but a senior captain, temporarily commanding a small number of ships, detached for any special purpose. He has no permanent rank, recognized by government, above his captaincy; though once employed as a commodore, usage and courtesy unite in continuing the title.
The practice was not reserved to captains in the earlier days. Captain Isaac Hull, chafing at not being able to progress further in rank, wrote in 1814 that, if no admirals were to be authorized, something should be done to prevent "every midshipman that has command of a gunboat on a separate station taking upon himself the name of Commodore."
Eventually the title of commodore was defined more strictly, and was reserved for captains so designated by the Navy Department, although the practice of retaining the title for life added some confusion. In 1857, Congress established the grade of Flag Officer. This generic title was intended "to promote the efficiency of the Navy," but differed little from the previous practice. Like the courtesy-title commodores, "flag officers" reverted to captain once their squadron command assignment was completed.
Because of the acute need for officers at the beginning of the American Civil War, naval tradition was ignored and commodore became for the first time a permanent commissioned rank. Eighteen were authorized on July 16, 1862. The rank title also lost its "line command" status when, in 1863, the chiefs of the Bureaus of Medicine and Surgery, Provisions and Clothing, Steam Engineering, and Construction and Repair were given the rank of commodore.
The rank of commodore continued in the Navy until 1899, when the Naval Personnel Act made all commodores into rear admirals. The reason, according to Laws Relating to the Navy, 1919, was "... on account of international relationships, the consideration of which caused the Navy Department to regard the complications confronting it as inimical to the honor and dignity of this nation, because of the adverse effect upon its high ranking representatives in their association with foreign officers." US commodores were not being treated as flag-level officers by other navies, or given the respect the Navy Department thought was their due.
As it would have been expensive to increase the pay of all the former commodores to the level of Rear Admirals, Congress specified that the lower half of the Rear Admiral list have pay equal to Brigadier Generals of the Army. If there were an odd number of Rear Admirals, the lower half of the list was to be the larger. All Rear Admirals, upper or lower half, were equal to major generals, flew a flag instead of a broad pennant, and were entitled to a thirteen gun salute. The Supreme Court held that the rank of Commodore had been removed from the navy, leaving it without a rank equivalent to Brigadier General. This act disgruntled Brigadier Generals, who could now be outranked by officers who were their juniors in terms of service. This was a point of inter-service controversy, and in 1916 the army made its Brigadier Generals equivalent to Rear Admirals (lower half). Thus, Rear Admirals (upper half) were equal to Major Generals, and Brigadier Generals were equal to Rear Admirals (lower half), but Major Generals still out-ranked Brigadier Generals.
During the naval expansion during World War II, the Navy Department was concerned that the appointment of more flag officers would create a glut of admirals after the war. However, some captains were holding commands of higher responsibility, and needed to be recognized. Admiral Ernest King proposed bringing back the old rank of commodore for these officers. President Franklin D. Roosevelt agreed, though he specified that this rank be restricted to line officers. The Navy's one-star officer reappeared in April 1943. In practice, staff corps officers could also become commodores. By the end of the war, there were over one hundred commodores in service. Very few of the wartime commodores were promoted to rear admiral. Promotions to commodore ended in 1947, and all had left the navy by 1950.
The one star rank appeared again in 1982 with the title of Commodore Admiral. The next year, after numerous protests to the Chief of Naval Operations, Commodore Admiral was changed to simply Commodore. In 1985, Commodore had also begun to appear as a position title for senior Captains who commanded Destroyer Squadrons, Patrol Boat Flotillas, and Naval Aircraft Groups. To prevent confusion between the title of Commodore and the actual rank, the one star Navy admiral rank was changed to Rear Admiral (Lower Half). From that point on, Commodore remained a title and all Navy one star admirals were referred to as Rear Admiral (Lower Half).
The Navy no longer maintains a rank of Commodore but the term has survived as a title. Modern-day Commodores are senior Captains in command of Destroyer/Cruiser Squadrons, Coastal Warfare Groups, and Aircraft Squadrons. Such officers are referred to, both verbally and in correspondence, as "Commodore," but wear the insignia of a Captain.
The German rank of Commodore (spelled Kommodore and pronounced Kom-o-door-eh) originated as a title used by some Captains in World War I. A German Commodore could hold any naval rank between Lieutenant and Captain and the title of Commodore was held by those officers who held tactical control over more than one vessel. This was most common with U-Boat commanders in charge of several submarines that were assigned to a single task force. (In the 19th-Century, German officers of this rank were referred to as Fleet Captains.)
By World War II, Commodore had at last become an actual rank in the Kriegsmarine (German War Navy). The position was considered that of a senior Captain, with insignia being the shoulder boards of a Captain with one thick rank stripe on the sleeve. German Commodores also were permitted to wear greatcoat lapels and visor insignia of an Admiral but were not officially members of the German Admiralty. The World War II rank of Commodore existed in a grey zone of seniority, very similar to the Schutzstaffel (S.S.) rank of Oberführer.
After the fall of Nazi Germany, and the rebuilding of the Deutsche Marine (modern German navy), Commodore fell into disuse as a rank, effectively replaced by the Flotilla Admiral (Flottilenadmiral) which is the lowest flag rank. It reappeared as a title in the 1950s. In the modern age, a German Commodore is a senior Captain who holds the rank of Captain but holds such positions as a Naval Squadron Commander.
Commodore is a title often held by an officer commanding a number of ships, or the senior captain within a shipping company. It is also a title held by the senior officer of many yacht clubs and boating associations.
In the Church of Scientology, "the Commodore" is one of many titles (and some critics would say, the least grandiose among them) held by the church's founder, L. Ron Hubbard. As the head of Sea Org, a pseudo naval branch of Scientology, Hubbard awarded himself this title. In fact, Hubbard never earned an actual rank higher than Lieutenant in his U.S. Navy service during World War II.
- Originally based on text by Justin T. Broderick, Salt Lake City, Utah.  Used with permission. 
- And on public domain information published by the US Navy.
- The Emperor's New Clothes