In the latter part of the 21st Century, mankind had finally accepted a Unified Government. Under the United Global Federation, mankind expanded their scientific knowledge to heights previously unimaginable.
Space exploration grew, and space-station based colonies were placed in orbit around the Sun.
As time passed, the Colonies became independent of the home, disdaining Earthlings to the point of hatred. Wars broke out between the Colonies and Earth, lasting 5 years. The surface of the planet was ravaged, racked with nuclear, biological and chemical weapons.
Eventually, the Colonies developed the means to travel at near-light speeds. Using this technology, the Colonies went in search of their own worlds.
The people of Earth, in order to survive, developed another passion. Instead of expanding outward, they expanded inward. Digging deep into the planet's surface, they built fabulous Cities. Each City held more than 50 million people, extending miles towards the center of the Earth and upwards, in domes miles high. Each City is an independent entity, capable of sustaining its own population.
Contact with other Cities dwindled to the point of phobia. Nobody is ever allowed inside a City for any reason. Criminals and Social rebels are expelled to the "Outside", forever banned from life in a City.

The Outside world has continued, but in an unnatural fashion.
Radioactive fallout and lingering effects from the Chemical and Biological weapons have affected the entire globe.
Some were able to escape the gene-reshaping combination of the weapons. They have become known as The Pure Ones.
Others were not so lucky. The terrible combination of Nuclear, Chemical, and Biological effects changed the very makeup of Species.
The DNA of almost every living thing was undergoing constant attack. Each generation was different than the previous. Some of the differences were minor, some drastic. Many did not survive. Each surviving generation produced a stronger successor.
Eventually, the changes started to become less random. The Animal kingdom branched onto two separate paths. One of the branches slowly reverted back to the point where they were virtually identical to their ancestors.
The other branch became self-aware. Each Species developed into an independent, distinct Race. Intellect varied widely, with some rivaling or surpassing The Pure Ones, and some not much more capable than a young human. Each Species also developed Racial Mutations, abilities never before seen in a particular Species, some of which are unique or specialized to their Race. These abilities range from the mundane to the wildly fantastic. They have become The Wild Folk to the general population.
Humanity was not spared from this change. The difference lies in the fact that the DNA changes were not Race specific. No two generations are identical. Offspring often have traits from the Animal kingdom, some have abilities only dreamed of. Some are grotesquely obvious, others cannot be told from a Pure One. These have come to be called The New Breed.
The New Era begun on the ashes of the Old.

N.E. 2107 - In the centuries since the Ancients destroyed the way the world was meant to be, their Cities exist, but none have entered them. Most of those who have been cast out of the Cities did not survive the world as it has come to be. Others have used their knowledge of the Ancient's devices to survive. These people are known in three ways:
Towns and cities have grown in the world since the Wars, and are populated. The smaller towns and villages are generally type specific, i.e. all Pure Ones, all New Breed, or all Wild Folk. The larger cities are populated with all three of the Gamma World residents, but caste systems have developed, ranging over many different levels in many different combinations. In one city, The Pure Ones may be the ruling class. In another, The New Breed. The Wild Ones rule in some places, but not many. There is the rare city where what you are does not matter.
Technology of the Ancients was still around after the War, but for several hundred years there was nobody to use it. Robots were plentiful, but the EMP of the War disabled many of them. Medical science could literally bring back the dead and fit humans with robotic parts, but only the computers and robots were used to perform these operations. The Stasis Field branched from the study of the Shield, so it is conceivable to find operating Artifacts, but extremely unlikely.
In most places, The Gamma World is a frontier. Dangerous, but very rewarding for the adventurous, the brave, the smart, and the lucky.