The Inner Colonies were the first worlds terraformed and colonized by the Unified Earth Government, relieving the overpopulation pressure on Earth that resulted in the Interplanetary War. Collectively, Earth and the Inner Colonies are known as the core worlds.
The Inner Colonies consisted of the 210 planets which were colonized before 2390. In 2490, Reach, which was located near the boundary of the Inner and Outer Colonies, became the UNSC's military center. As the UNSC began settling the Outer Colonies, the Inner Colonies, with their established social and economic infrastructures, became the political and economic basis of the UNSC, supplied with raw materials from the newer settlements.
It is known that there was a war some time before the 26th century called the "Inner Colony Wars". Little is known about this conflict, except the fact that unrestricted news reports during the war caused "massive civil unrest" and threatened to "tear down Earth's culture".
During the Human-Covenant War, numerous ships were pulled from their positions in the Outer Colonies to defend the Inner Colonies. Old ships, such as the Halcyon-class light cruiser UNSC Pillar of Autumn, were refitted and brought back into service to supplement the remaining defense forces. At the beginning of the Fall of Reach, dozens of Inner Colony planets were bypassed by the Covenant fleet attacking Reach, and a number of Inner Colonies survived the war.
 Known Inner Colonies
- Reach - Glassed in 2552, reterraformed by 2589
- Circumstance - Unknown status, likely glassed
- Tribute - Glassed in 2552
- Beta Gabriel - Unknown status, likely glassed
- Tantalus - Unknown status, likely glassed
- Earth - Under UEG control
- Mars - Unknown status, likely under UEG control
- Jovian Moons - Unknown status
- Pluto - Under UEG control
- Chi Rho - Unknown status
- ^ The presence of orbital defense platforms around New Carthage implies that it is an Inner Colony, though this has not been confirmed.
- ^ Halo: Evolutions - Essential Tales of the Halo Universe, "The Impossible Life and the Possible Death of Preston J. Cole", page 429
- ^ Halo: Contact Harvest, page 33
- ^ Halo Encyclopedia (2011), page 43
- ^ Halo Graphic Novel, Second Sunrise over New Mombasa
- ^ Halo: The Fall of Reach, page ??
- ^ Halo: First Strike (2010), Adjunct: Petra