- "You've been working in the same sectors too long, Jack...makes for comfortable pirating but narrows your view of the universe."
- ―Han Solo
Groupings of inhabited systems organized themselves into informal sectors from before the Infinite Empire. The Galactic Republic was the first body to attempt a large scale application of standardized sectors. A sector was defined by the Republic as having a maximum of 50 inhabited star systems to keep sectors at a manageable level. 
The rapid exploration, expansion and colonization beginning around 5000 BBY caused many sectors to grow larger than the intended size. For instance, in 19 ABY, the Remnant controlled only eight sectors, which were said to contain "over a thousand inhabited systems", which would indicate an average size of 125 inhabited systems per sector. At some point in the history of the galaxy, there were millions of sectors, as populations grew rapidly. The Ruusan Reformation of 1000 BBY reorganized the galaxy into 1,024 regional sectors, which were divided into even smaller constituent subsectors.
The Ruusan Reformations were problematic however, as they created a moratorium on the creation of new sectors, meanwhile beings would continue to inhabit worlds beyond the borders of the established Republic. This led to annexation and overinflation of existing sectors. For example, the lightly populated Chommell sector encompassed 36 full-member worlds, more than 40,000 settled dependencies and 300,000,000 barren stars in the final years of the Republic. Chancellor Palpatine reversed the moratorium on new regional sectors and created new sectors by dividing up larger ones. Any freestanding subsectors were grouped up into a single sector.
By Republic law, in state of emergency, an appointed governor-general coordinated military actions with the Senator of a troubled sector. This law was invoked many times during the Clone Wars by Supreme Chancellor Palpatine.
When the Galactic Empire took control of the Republic's sectors, Moffs replaced senators as sectorial governors. Sectors were also commonly expanded and collapsed together to consolidate power in the officers delegated by Palpatine to rule. Any unincorporated territories wishing for inclusion to the Empire became new freestanding subsectors, and were added to new or existing regional sectors as events warranted.
Many sectors owed their name to their primary system which in turn was named after its primary planet. For example the planet Alderaan belonged to the Alderaan system and thus, to the Alderaan sector.
Maps of the sectors of the Outer Rim, the Mid Rim, and the Expansion Region, as reorganized by the Galactic Empire: