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The term Synth refers to a class of alien creatures that have been synthetically cloned and turned into war machines, and one of the several species that make up the Combine organization.

They are officially described as "self-replicating robots that evolve", "created or taken over by the Combine and enslaved during their conquests and wars."[1]

All Synths seem to possess both organic and robotic characteristics, suggesting that they're the synthetic derivatives of alien species that were previously found by the Combine, and modified to their needs.[2]


Crab Synths and Mortar Synths being transported on a conveyor belt.

Synths always appear as average to large insect-like or arthropod-like creatures, usually with segmented legs or antennae-like sensory structures, resembling locusts, crabs or similar beings.

Even though Synths possess organic characteristics, they are fully artificial, as their name suggests. They are shown to possess a brain, however, it is most likely synthetic and derived from the original creature as to perform the functions of a real brain.[2][1]

It is unknown how many of the Synths presented in the series are 'true' to their initial species or forcefully evolved by the Combine, since the Combine modify their appearance by adding mechanical equipment and weaponry. It is also unclear whether they all come from the same planet (mirroring the Xen creatures that were also brought from other worlds and, for some of them, also enslaved). The Strider, however, appears to have a brain, as seen in Episode Two. This is the only known Synth to possess any organ at all, however, other Synths likely possess one too.

The City 17 Citadel features factories where Synths are manufactured. For instance, Stalkers can be seen working on Gunships in the lower levels. Synths are also never observed to be "self-replicating" in-game, but seem to be raised and/or manufactured by the Combine.

Their carapace-like armor is largely immune to small arms fire (with the notable exceptions of the Hunter and Shield Scanner) and bullets striking it produce a metallic 'ping' noise. These outer shells can be ripped open with enough force, as demonstrated when Dog tears a Strider's head open in Episode Two. The only weapons that are known to be effective against the larger Synths are explosives (such as the RPG, the MP7's secondary fire, or the Magnusson Device) and the Combine Energy Ball.

Synths may or not have been used by the Combine during the Seven Hour War. Forces as the human-derived Combine units, such as Overwatch Soldiers, Hunter-Choppers, and APCs, would have only started appearing after the Combine rule on Earth had been established.

It is likely that the Combine forces used to invade earth were far more advanced than those stationed on earth, such forces being constituted of synths or not.

Known Synths

Dog attacking a Strider in the Outlands, as seen in the second Episode Two teaser trailer.

Various types of Synths have been brought over to Earth by the Combine, and more types of Synths exist elsewhere. Some, such as the Shield Scanner, are merely used as scouting or transport and have little to no weaponry, while others, such as the Strider, are fierce war machines, however, its worker version is adapted for work and possess no weaponry. The known Synths are:

Behind the scenes

Combine Gunship factory in the Citadel.

  • According to Dhabih Eng, the team tried to incorporate elements that were neither strictly organic nor purely mechanical to suggest their original organic forms. They did not want to have giant alien creatures with armor attached or implants added, but rather show a forceful evolution of the equipment unnaturally imposed on them by the Combine. In the end, the designs intentionally blurred the line between machine and animal, in their appearance and more importantly their motion and how they reacted to the world around them.[1]
  • According to the book Rising the Bar, the idea was to start Synths as androids or synthetic creatures that have been enslaved by the Combine through conquests and wars. Mentioned as "Self replicating robots that evolve", instead of having to be maintaneed, repaired or upgraded, they would make a new version of themselves every five years. Ted Backman says Synths were inspired by the android Bishop, featured in Aliens.

Cut Synths

Several Synths were cut from the final game:

Early Synths

Several pre-release materials gives some insight about the early Synths.

"Weather Control Vignette"

Raising the Bar features a story fragment of the Weather Control chapter given to the developers for design inspiration, relating parts of the Weather Control battle, with early names for the Combine units. For example "mech" seems to stand for the predecessors of the Synth, while "mech carrier" likely refers to the Combine Dropship. The text depicts the mech looking like Dog's cousins, but apparently new-minted and flawless, and with a "head" in quotes, suggesting that it is strangely-looking. Also, at some point in the text, its head pulses with a faint glow as if powering up for energy, then fires, which is very similar to the Strider's warp cannon, added to the fact that it is brought by a "carrier", the Strider being brought by Combine Dropships without their containers. Added to the gun located in its head, it also has mounted guns.[1] This "mech" could be an early version of one the Synth predecessors of the Overwatch Elite, the Combine Synth Elite Soldier, which has been through several iterations with strangely-looking heads and a narrow waist like Dog (as in the picture seen here), or the Combine Super Soldier, also with a strangely-looking head.


A folder called "TBot1" can be found in the model textures files from the playable Half-Life 2 Beta files, in a folder called "obsolete". These are likely the textures for the early "mech" or (ro)bot from the Weather Control story fragment, a very early NPC, since they possess some similarities with Dog in design, mirroring the Weather Control story fragment covered above. Since Dog is said to be one of the few concepts that went from the first sketch straight into the game with relatively few changes to the design,[1] it could be assumed that the Weather Control mechs are two-legged and bear similar colors and body part plates to Dog, since he was made from scraps from different metal sources, such as Combine metal (an old texture file for Dog also shows a "15" like logo also found in the TBot textures).

The textures also suggest a slightly battered robot and show round eyes that recall the City Scanner's eye (Dog's head). They also show that a yellow color scheme was to be used, and suggest that a bipedal, paw-footed design like Dog's was also to be used. The model is however missing, and it is next to impossible to guess its shape from its texture files. The "1" in the folder's name also suggests that there were probably other versions, (such as non-battered conditions or other colors). Other texture files suggest that TBot had an antenna and was equipped with a flamethrower, and that some of its internal circuits were to be visible and blue, like the early Combine Dropship, the Crab Synth and the Mortar Synth. The files also suggest a "collar" made of green fabric, with the same fabric on one shoulder and the thighs, showing another example of the old concept of the Combine recycling human materials.

The TBot ("bot" implying "robot") seems more mechanical than other Synths found throughout the universe. It appears to sport several cogwheels and pistons on its body, more in line with the "Combine recycling human materials" mood of the very early Half-Life 2 storyline. Overall, the TBot design appears to be much closer to the "mech" from the cut Weather Control story fragment, making it a likely an ancestor of larger, two-legged Synths like the Combine Synth Elite Soldier or the Combine Super Soldier.



A variant of the TBot1, with green eyes, the colour of the robot is unknown, because the TBot in concept art is obscured.

All existing TBot 2 files were deleted from the retail version of HL2.

"Alyx Vignette"

A story fragment from Raising the Bar is set at the start of the Air Exchange chapter and has Gordon involved in a train wreck. Alyx, accompanied by her pet Skitch, encounter Gordon before being attacked by "tripods" (i.e. Striders) and "Combine Elite": tall, metallic shapes making muffled, almost silent movements. The description does fit that of the Combine Super Soldier (named "Combine Elite" in Raising the Bar[1]) while also vaguely matching the Combine Synth Elite Soldier, another elite Combine unit. Also stated in the book, the Elite was turned into the humanoid unit seen in the final version.


The Gunship back logo.

Most Synths bear at least one logo. Gunships and Striders bear the same logo on their back and head, respectively. This logo was on the back of the Overwatch Soldier's old texture (in white) and can also be seen on the shoulder of one of the Combine Synth Elite Soldier versions. Its meaning and relation to the Combine are unknown, as are the possibilities of it proving any relations between the Strider and Gunship. Other Synths such as the Shield Scanner, the Dropship and the Advisor Pod bear the standard "claw" logo (in dark red for the first two and white for the latter), while the Shield Scanner also bears a trimmed "CMB" logo (like the City Scanner). The Mortar Synth, Crab Synth, and Hunter bear no logo at all.

Beta Overwatch Soldier version.

As for the cut Synths, the Wasteland Scanner bears the standard "claw" logo (in dark Red), and other unknown markings, partially seen in the Overwatch Soldier outfit. The Combine Guard has no logo, while its predecessor, the Combine Synth Elite Soldier, has some informal logos in some of its versions, as well as the Gunships / Strider logo, as stated above. The Sacktick has two informal logos, one on its back, one on its right front leg, not seen elsewhere. The Combine Super Soldier has the number "314" in its concept art (also featured in the Beta Overwatch Soldier outfit), and two Red triangles in a Red-rounded circle, reused for the Overwatch Soldier outfit, although with different colors. It also has Red strips in several locations.


  • The lack of organic matter left behind when Synths explode suggest that they are mechanical in nature.
  • The term "Synth" relates to words such as "synthetic", "synthesis" or "synthetical". A "synthetic" element is "made artificially by chemical synthesis";[3] a "synthesis" is "the combining of separate elements or substances to form a coherent whole";[4] and a "synthetical" element is the combination of "separate elements to form a coherent whole".[5] The term "synth" could therefore be interpreted as a short for "synthetic". In a general science fiction sense, "synth" is often synonymous with words like "android", "robot" or "artificial (human)", namely in IPs like those belonging to Philip K Dick and the Fallout series of video games.
  • It could be argued wether Hunters are Synths; their vulnerability to gunfire and integrated armor (instead of a full metal body), make them more like cyborgs than Synths. Additionally, a blood-like substance is observed when they take damage, also suggesting organic matter.[6]


Retail Synths

Cut Synths

List of appearances


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