The Tau, also known as the Xi'll by the Second Human Empire are the central figures of the Tau Empire, an interstellar polity which is composed of several different intelligent species, primarily the Kroot of Pech, the Vespid of the world of Vespid and the nomadic Nicassar, though there are now several human Tau Septs derived from conquered Imperial humans or humans who voluntarily joined the Tau Empire because they were impressed by the concept of the Greater Good. These people are known as Gue'vesa in the Tau Lexicon and they are considered amongst the most vile of Traitors and Heretics within the Imperium of Man. The Tau are a relatively young race (it has been only 6,000 Terran years since Imperial Inquisitors first noted that the Tau had only just mastered fire and the wheel), and they have evolved rapidly over the past few millennia. Unlike other young intelligent races of the galaxy, the Tau have made remarkable leaps in technology and now is apart of the Alliance of Nations fighting against the Axis of Empires and the Coalition of Independent States.
Society and Culture 
Each alien race within the Tau Empire has a unique and different culture and society. However, all of the Tau Empire's races hold a common belief in the Greater Good that binds them together.
Tau society is centred around the philosophical concept of the Greater Good, where individual Tau sacrifice their personal gain in favour of benefiting the Tau Empire as a whole. It is from this philosophy that the Tau's all-purpose motto, "For the Greater Good!", is derived. The Tau castes, although different in role and organisation, work together for the benefit of all Tau: the Earth Caste provide and produce, the Water Caste communicate and distribute, the Air Caste connect the worlds of the Tau Empire, and the Fire Caste protects and defends all the others. The external image the Tau project towards other species is that they are enlightened, altruistic and idealistic, believing in the unification of all "for the Greater Good".
Tau society is divided into five castes:
- Fire Caste - The military ground forces of the Tau Empire
- Earth Caste - The engineers, laborers and scientists of Tau society
- Water Caste - The diplomats, storytellers, administrators and bureaucrats of Tau society
- Air Caste - The pilots and officers of the Tau Navy
- Ethreal Caste - The political and religious leaders of the Tau Empire
T'au ground warfare is carried out almost exclusively by the Fire Caste, whilst the Air Caste is responsible for aerial and space combat, and providing transport between star systems for the Fire Caste. Fire Caste military forces are known as Hunter Cadres, and are similar in size and battlefield role to an Astra Militarum company and Alliance Joint Military.
Hunter Cadres are primarily made up of T'au from the same planetary sept. Auxiliary forces are drawn from the alien member species within the T'au Empire, and Hunter Cadres can include Kroot warriors, Vespid Stingwings, and HUman Gue'vesa amongst other intelligent species.
The T'au prefer long-ranged combat over close melee, disdaining the latter and viewing it as primitive and barbaric. A T'au Fire Warrior's primary weapon is the Pulse Rifle, which can be switched for a Pulse Carbine. A Pulse Rifle fires energetic particles which quickly break down and are projected as a "pulse" of superheated plasma. Pulse Carbines, also carried by Fire Warrior teams, sacrifice range for portability, but incorporates an underslung Photon Grenade Launcher.
The Photon Grenades are powerful defensive weapons that not only release a powerful burst of light, but also a sonic shockwave that can temporarily stun the target. As the T'au Empire integrates more and more alien races, they use whatever unique abilities they possess in supplementing T'au tactics.
Often small alien squads will be used as bait to draw enemy fire, though it is important to note that bait squads are not considered expendable but rather dependent on the effectiveness of their cover fire.
If T'au forces ever do become engaged in close melee combat, they can call on their cannibalistic allies, the Kroot. The Kroot are expert jungle fighters who appear deceptively slender.