Combine Overworld

Very little is known about the Combine Homeworld, as it has never been directly mentioned in any of the Half Life games. However, it is safe to assume their Homeworld has some connection to Xen, as the Combine use Headcrabs as weapons in Half Life 2.

What little information or speculation that can be made suggests that the Combine Homeworld's climate is very different from that of Earth, as all Combine species seen so far feature a sort of breathing apparatus (The Advisor has a small black cup over its snout and the Combine overwatch all feature obvious masks). This suggests that the Combine Homeworld does not support Oxygen or Methane, which is apparent in Earth's atmosphere.

This raises interesting questions as to why the Combine are interested in occupying Earth. One answer to this is Combines apparent need to drain Earth of its resources.