Clauses:
GSC Ubers, OU, UU and LC all use the following clauses:
Sleep Clause
Species Clause
Self-KO Clause
OHKO Clause - You cannot use the moves Fissure, Guillotine or Horn Drill.
Evasion Clause - You cannot use the moves Double Team or Minimize.
Sleep Trap Clause - You cannot have a sleep inducing move (e.g. Hypnosis) and a trapping move (e.g. Mean Look) on the same Pokémon.

All Pokémon listed in a tier are banned for all tiers below the one they are listed in.
LC tiers are completely seperate from the main tier list.
You can only use items, abilities and Pokémon that are officially released by Nintendo in our tiers

GSC Ubers:

Spoiler

Celebi, Ho-Oh, Lugia, Mew, Mewtwo

GSC OU:

Spoiler

Aerodactyl, Alakazam, Articuno, Blissey, Charizard, Clefable, Cloyster, Donphan, Dragonite, Entei, Espeon, Exeggutor, Forretress, Gengar, Golem, Heracross, Houndoom, Jolteon, Jynx, Kangaskhan, Kingdra, Lapras, Machamp, Marowak, Meganium, Miltank, Misdreavus, Moltres, Muk, Nidoking, Porygon2, Raikou, Rhydon, Scizor, Skarmory, Smeargle, Snorlax, Starmie, Steelix, Suicune, Tauros, Tentacruel, Typhlosion, Tyranitar, Umbreon, Ursaring, Vaporeon, Venusaur, Zapdos

GSC UU:

Spoiler

All Pokémon not listed in tiers above are allowed.

GSC NU:

Spoiler

GSC NU is not playable on the main PO server. However, this list of Banned Pokemon has been provided so users can play the tier if they so wish.

Banned Pokémon: Ampharos, Arcanine, Bellossom, Blastoise, Chansey, Crobat, Dodrio, Electabuzz, Electrode, Feraligatr, Flareon, Girafarig, Gligar, Golduck, Granbull, Gyarados, Hitmonlee, Hypno, Jumpluff, Kabutops, Kadabra, Lanturn, Magmar, Magneton, Nidoqueen, Ninetales, Omastar, Piloswine, Pinsir, Politoed, Poliwrath, Primeape, Quagsire, Qwilfish, Raichu, Sandslash, Scyther, Slowbro, Slowking, Victreebel, Vileplume, Weezing, Wigglytuff

GSC LC:

Spoiler

Only Pokémon that have not yet evolved but are capable of evolution are allowed in this tier.

Banned Moves: Dragon Rage, Sonic Boom

Additional Notes:
Hidden Power Legendaries were banned on NetBattle, but that was mostly due to the fact that the RNG could not be cracked back then. As seeing how we've cracked the RNG for all the modern games, GSC should be a piece of cake. This is why we have decided not to implement the ban.

If you believe HP Legends should be banned on the basis of other criteria, then feel free to start a discussion thread in the Past Gens Discussion forum.

Thanks go to borat for the OU/UU list.

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GSC OU Viability Rankings: http://pokemon-online.eu/threads/28300/
borat's Guide to GSC: http://pokemon-online.eu/threads/6541/