From Command and Conquer Wiki

The concept of an all terrain battle walker, capable of delivering heavy firepower from any location on the battlefield, has first surfaced in the First Tiberium War in the form of a prototypical bipedal X-O powersuit. The versatile unit was lightly armoured, but could carry several different weapon systems, which included a 35mm rocket launcher and a high-powered laser. The results of initial tests were so promising, that the technology was extensively researched and developed.

Walkers formed the backbone of the GDI taskforce in the Second Tiberium War, following aforementione research and development. The Humvee was superseded by a Wolverine Powered Combat Suit with twin automatic cannons on its arms with recon as its role. The M1 Abrams was replaced with the Titan Medium Mechanized Battle Walker with an 120mm APDS side-mounted cannon. The Titan had a fairly long range and was often used to take out Nod defenses beyond their range. However, the technology was still being further developed, as the mighty Mammoth Mk. II was being developed and first units fielded at the time.

Following the events of the Second Tiberium War, GDI was expanding on the concept, developing another walker unit, the Juggernaut, equipped with three 155mm naval cannons, as a countermeasure to Nod artillery. CABAL also invented the Core Defender.

However, the walker technology had its drawbacks. The mechanisms utilized by these units were not only expensive and complicated to build, they also required constant maintenance and were relatively weak and fragile in the face enemy fire, as well as being unreliable in the field. Another weakness was their susceptibility to explosives when placed on legs. As such, the frontline walkers were gradually phased out and replaced with cheaper, more resilient tanks, such as the MBT-6 Predator and Mammoth 27, with only the long-range Juggernaut Mk. III remaining in general service in 2047.

Ironically, Nod managed to overcome many of these drawbacks. First, the Black Hand under Marcion developed the Purifier, equipped with an Obelisk-derived laser, as well as flame weaponry and a subliminal influencing device. Following the resolution of faction strife, the Purifier was succeeded in general Nod use by the upgradeable Avatar Warmech. The Avatar packed a considerable amount of firepower in its scaled-down Obelisk laser and was sufficiently armoured to take on a large number of smaller ground units. However, it suffered once again from being costly and vulnerable to attacks from aircraft, but compensated it to a degree with it's upgrade capability. A development of combining the the Avatar technology with GDI's Mammoth Armed Reclamation Vehicle project is Nod's Redeemer.

When the Scrin invaded Earth they brought their own walkers with them. However, the technology behind Scrin walkers may be considered far removed from traditional human walker technology, involving bio-mechanical constructs that blur the line between machine and creature. While a number of Scrin units 'walk' on multiple insect-like legs, the only Scrin units to be considered walkers in the traditional sense is the Annihilator Tripod, which appears the most mechanical and the even more massive Eradicator Hexapod.

	Mechanized walkers
	GDI: X-O Powersuit, Wolverine, Titan, Mammoth Mk. II, Juggernaut, Behemoth Nod: Avatar Warmech, Redeemer CABAL: Core Defender Scrin: Annihilator Tripod, Eradicator Hexapod