Let this war end in either total victory or our extinction. No further compromise shall we allow. We take our stand beside you, here, upon this miserable rock.

The Combine Empire sprawls across multiple parallel universes, and includes an unknown number of sentient species. Details of the Combine's leadership and administrative structure remain unknown. On Earth, aliens called the Advisors are responsible for facilitating all communication and interaction with the Human race.

The Combine manipulates species through a variety of means, including bioengineering and artificial implantation. The products of the Combine's re-engineering process are known as "synths", bio-mechanical organisms that serve a variety of functions within the empire. The Combine also uses their technology to create armies of super-soldiers, uniquely adapted to the particular environments of the conquered worlds. This process results in a highly mobile and adaptive military force which is able to respond to any threat and crush any opposition.

Vocal stress is placed on the first syllable. The name is pronounced COM-bine, as in Combine harvester, as opposed to the verb form com-BINE, as in "to join together or unite." Dr. Breen refers to the Combine Soldiers and Civil Protection as "The Transhuman Arm of the Combine Overwatch" [2] in Nova Prospekt, verifying that "Combine" is an accepted name of the empire, even by the empire itself.

Vocal stress is placed on the first syllable. The name is pronounced COM-bine, as in Combine harvester, as opposed to the verb form com-BINE, as in "to join together or unite." Dr. Breen refers to the Combine Soldiers and Civil Protection as "The Transhuman Arm of the Combine Overwatch" [2] in Nova Prospekt, verifying that "Combine" is an accepted name of the empire, even by the empire itself.

It is unknown how many worlds the Combine has under its control; it is known that in addition to Earth, the Combine invaded the home-world of the Nihilanth's species. The defeated Nihilanth escaped the onslaught and sought refuge in Xen.[4] It brought with it members of a number of enslaved or allied species, including the Vortigaunts.[4][5]