How do I transfer previous generation Pokémon to X/Y?

Things you'll need to go from Generation 3 --> 6 (aside from games)

• Nintendo DS, or Nintendo DS Lite (contains both a GameBoy Advance game pak slot, and a Nintendo DS game card slot)
• A Second Nintendo DS to transfer Pokemon from generation 4 --> 5
• A Nintendo 3DS to transfer Pokemon from generation 5 --> 6. If you have a 3DS, you don't need the DS from the last step.

Generation 3 --> 4

Transfer up Pokemon using PalPark from Ruby, Sapphire, FireRed, LeafGreen, and/or Emerald to Diamond, Pearl, and/or Platinum. For this you need an original Nintendo DS or a Nintendo DS Lite as you need access to a Nintendo DS game card slot, and a GameBoy Advance game pak slot. Additionally, you can not transfer up eggs.

Gamecube Games --> Gen 3

You can also transfer up Pokemon from Pokemon Colosseum or Pokemon XD: Gale of Darkness to a modern Pokemon game, you just need to transfer them via trade to a generation 3 game, and then transfer them up like normal Pokemon.

Generation 4 --> 5

At Route 15 in Black or White you'll have to transfer everything up from your generation 4 games. For this, you'll need 2 Nintendo DS systems, one following the instructions in Pokemon Black, White, Black 2, and White 2, and another with the generation 4 game inserted running DS Download Play. You can transfer up Pokemon in this fashion, however there are a few stipulations:

• Held items will not be transferred up to generation 5 (they'll automatically be returned to the player's bag)
• The Pokemon being transferred can not know any HM moves.
• You can not transfer eggs.
• The Spikey-eared Pichu (a promotional exclusive) can not be transferred up.

Generation 5 --> 6

Starting December 27th 2013, download the 3DS application titled Pokemon Bank from the Nintendo eShop. This application can take Pokemon from a Generation 5 or 6 Game, and store them inside of the app. After this process is done, withdraw the Pokemon from Pokemon Bank in your generation 6 game.

Things to note when transferring Pokemon

• All Pokemon are forever lost from their past generation game - there is no way to transfer a Pokemon back to its previous generation.
• Game mechanics change between generations. While a team might be effective in one game, it might not prove to be useful in another. Notably moves are assigned physical or special in generation 4 onwards; and generation 6 introduces new mechanics such as the fairy type, base stat changes, and move changes.

As a detail explanation of Gen 5 to Gen 6: You'll need 1 3DS, 1 Gen 5 game, Poke Transporter, Pokemon Bank, and 1 Gen 6 game.

First thing you'll want to do is go to your Gen 5 game. This is Black, White, Black 2, or White 2. You'll want to put the Pokemon you wish to transfer in the first box. It must be the first box in the PC, or Pokemon Transporter will not take it.

From there, go to Transporter. You will be warned that items will be sent back to your bag, and if your bag is full they will be lost. You will then be prompted to confirm.

After your Pokemon have been put into the Bank, switch games to your Gen 6 (X or Y) that you want to put them on. Open the Pokemon Bank app, and use the bank. If you go one box over from the Box 1 of the bank, you will see Transporter (Or Transfer, I don't remember the exact wording) Box. These are the Pokemon you transported in.

When you put these Pokemon in your box, they go straight there. You do not need to do Pokemon Link on your Gen 6 menu as you do for gifts (Such as Celebi).