The series features several sentient alien races. The most important to the books are the insectoid Priest-Kings and the huge, sharp-clawed, predatory Kurii, both spacefarers from foreign star systems. The Priest-Kings rule Gor as disinterested custodians, leaving humans to their own affairs as long as they abide by certain restrictions on technology. The Kurii are an aggressive, invasive race with advanced technology (but less so than that of the Priest-Kings) who wish to colonize both Gor and Earth. The power of the Priest-Kings is diminished after the "Nest War" described in the third book and the Priest-Kings and Kurii struggle against each other via their respective human agents and spies.