The Inner Sphere is a fictional region of interstellar space surrounding Earth to a radius of roughly 500 light years created for the Battletech Universe. The five Successor States, each ruled by an ancient Great House, are the Inner Sphere's dominant power in the setting's "modern" era, although other nations, empires, and independent worlds have ruled parts of the Inner Sphere in the past, and continue to exist. The Inner Sphere has been fractured as each of the Great Houses attempted to secure rulership during the Succession Wars.
The five Successor States are the largest political parties in the Inner Sphere. The Sphere was divided into five "pie wedges" around a central, roughly circular core: the five Great Houses and the central Terran Hegemony. The Great Houses divided the Terran Hegemony piecemeal among themselves, and have had a series of wars for supremacy, known as the Succession Wars.
The political structure of the Successor States is essentially feudal, with the exception of the Free Worlds League, which has a federal structure, at least at the highest levels of government. Planetary governments in the Inner Sphere span the entire range of political structures, from democracy to dictatorships, although authoritarian forms of government are much more common.
At various times, other political entities such as the or the independent worlds and small states of the Chaos March have shared power with the Great Houses, often acting as buffer zones between their more powerful neighbours.
Surrounding the inner sphere is an area called the Periphery which is home to a few nations of moderate power and independent worlds cut off from civilization by the collapse of the Star League. The Inner Sphere sees the Periphery as a backwater. The Periphery is also a safe place for pirates, criminals, and refugees from the Succession Wars.