The Imperial Republic is the dominate interplanetary government in the known universe. The nominal head of state is the Emperor of the Republic, but the real power is in the hands of the Imperial Parliament, which is divided into the Regular Council and the Council of Twelve.
Galactic Primus is the capital of the Republic.
The formation of the Imperial Republic was largely the desire of humans for a united interplanetary government to deal with Orks and other potential alien threats.