The Aliens are a series of Reptile-type cards that were introduced in Power of the Duelist with further support added in Gladiator's Assault and Light of Destruction with more upcoming in Crimson Crisis. Alien decks focus heavily on the use of "A-Counters" which weaken opposing monsters battling Aliens and allow the Alien user to take control of them. This is one of the first true Reptile Archetypes and also one of the only archetypes that focuses this heavily on manipulating the opponent.
Aliens require a high degree of skill to use properly, since they focus on manipulating the opponent rather than strengthening themselves. They have a wide range of lower-level monsters that can be used to oppress or manipulate the opponent but are lacking at higher levels; however, the ability to take control of an opponent's strong monster more than makes up for this deficiency. Duels involving Aliens tend to be a unique experience for both players, due to how quickly the balance of power can shift during the duel depending on how each player responds. However, with the new cards from Crimson Crisis, aliens have become much stronger due to the powerful field-clearing effect of Cosmos Fortress Gol'Gar and the rapid A-Counter distribution of Alien Revenger.
Aliens are at their weakest against opponents capable of either summoning overwhelmingly powerful monsters early, before they can be weakened by A-Counters, or against opponents who can remove their monsters from the field or flip them face-down again, such as a "Rock"-based deck or the popular "Gladiator Beasts." Razor Lizard can help take out some of these. No monster in the Alien Archetype has over 2600 ATK, and the two strongest monsters in the archetype technically do not count as "Aliens," so they gain no personal benefit from the weakening effect of A-Counters during battle. Evil Dragon Ananta can help in this cause. An Alien user needs to be aware to rely as much upon their "weaker" monsters as their stronger ones in order to prevail. Damage = Reptile and Snake Whistle helps to swarm.
Aesthetically, they are based on two sets of popular alien conspiracy theories: the "Roswell Grays" in Area 51 (for the "lower" Aliens) and the Reptilian Humanoid alien conspiracy (for Alien Hunter, Alien Warrior, Alien Shocktrooper, and Alien Mother); the second conspiracy may also have influenced the Alien's type as being Reptile, instead of Fiend or Fairy like other alien-like monsters such as Greed Quasar and Zeta Reticulant.
Most of the aliens have orbs, usually coloured according to their attribute, on their bodies.
Putting A-Counters on opposing cards is essential to the success of Alien decks, as Aliens generally have no extremely powerful monsters other than Cosmic Horror Gangi'el and Cosmos Fortress Gol'Gar (which do not strictly count as "Aliens").
Since counters cannot be placed on face-down monsters, generally the best way to ensure that A-Counters can be used against the opponent is to use Light of Intervention, though note that doing so negates much of the usefulness of Alien Grey. Other alternatives include such cards as "Ceasefire" or "Swords of Revealing Light". Some Alien users also use partial stalling effects like "Vengeful Bog Spirit" or "Stumbling" to ensure that they have enough time to place A-Counters on the opponent's monsters during their own turn.
According to the current ruling on A-Counters, they do not reduce the ATK and DEF of monsters battling Alien monsters if no cards exist on the field that mention that effect. This means that if you only control an Alien Shocktrooper, for example, your opponent's monsters will not be weakened no matter how many A-Counters are on them. The upside to this ruling, however, is that these effects stack, so if you control two Alien Warriors, the opponent's monsters will be twice as weakened for every A-Counter on them.