|Governing body||Re'tu Central Authority|
|Key episodes||"Show and Tell" (first appearance)|
The Re'tu are a non-humanoid race whose molecules exist 180 degrees out of phase from our reality, which prevent humans from seeing them. They can only be interacted with physically to a limited degree, although they can see humans perfectly. The Re'tu can become visible when exposed to radiation emitted from a Transphase Eradication Rod. The species are insectoid in appearance, resembling spiders with large eyes on the sides of their head, and spiny green arms and legs, permitting them to move quickly. They have multiple legs, and adults are approximately eight feet (two and a half meters) tall.
Because they are partially out of phase, Re'tu are highly resistant to physical weapons, which only "slow them down" (While not expanded upon, it is logical to assume that this means that physical weapons cause damage to the Re'tu in a way similar to the difference in how a bullet can travel through air compared to water). Re'tu are quite vulnerable to their own weapons as well as the energy beams fired by a TER device, and a single hit from either will cause them to explode messily.
Re'tu technology is fairly advanced. They possess powerful arm-mounted energy cannons that burn large holes through both flesh and metal, can create extremely powerful (as strong as a small nuclear bomb) yet quite portable explosives, and are able to create genetically engineered humans (namely the child ambassador nicknamed Charlie though his body was defective in several ways because of the Re'tu's limited knowledge of human physiology).
Because of their invisible nature, the Re'tu were deemed a threat by the Goa'uld, who attempted to exterminate them. Re'tu also emit a "vibe" that causes severe pain in a Goa'uld symbiote, which allows Goa'uld and Jaffa to detect their presence. 
The aggression of the Goa'uld has split the Re'tu into two opposing factors: the pacifistic Re'tu government who are non-hostile towards humans, and a splinter group of Re'tu rebels. These Re'tu rebels believed that the only way to fight the Goa'uld was to destroy all their potential hosts, including all human beings living in the galaxy. These rebels operated in 5-man suicide units, which infiltrated a target area with the purpose of and causing as much damage as they can before setting off an explosion equivalent to a small tactical nuke.
Encounter with the SGCEdit
As the Re'tu government knew about the plans of the rebels, they sent an envoy to Earth in the form of a human boy, capable of communicating with both his Re'tu "mother" and humans. The boy, who was subsequently named "Charlie," was sent to warn Earth about a threat from a Re'tu rebel faction.
After the Re'tu had sent their envoy, the SGC decided to gather intelligence about the new threat. Assisted by the Tok'ra, who provided the Tau'ri with several TER's, SG-1 surveyed the rebel's home base before returning to Earth. However, a suicide unit consisting of five Re'tu had made it back to Earth along with the returning troops, which almost destroyed the facility before they were killed by security teams. 
The Re'tu only actually appear in one episode, but they are mentioned several times throughout the show. Whenever something bad happens in plain view yet no visible cause is apparent, the invisible Re'tu are usually amongst the prime suspects.
Though not a canon appearance, the Re'tu were scheduled to appear in the cancelled video game Stargate: The Alliance.