Trade

Trading is one of the ways that one can complete the Pokédex. The trainer can use trading to get other Pokémon that they would like, or use trading to evolve certain Pokémon.

Process

There are several ways to trade Pokémon between games.

1. In Generation I and II, Pokémon can be traded between games of the same generation from the Link Club. The Link Club can be found on the 2nd floor of any Pokémon Center. 
2. Pokémon from Gen. II games can be traded to Gen. I games using the Time Capsule, which can be found on the 2nd floor of any Pokémon Center.
3. Generation III games cannot trade with games from Generations I or II. This is due to the game mechanics being completely revamped in Gen. III games.
4. In Generation III, the games set in similar regions can trade via the Wireless Club on the 2nd floor of the Pokémon Center. However, in order to trade between FireRed or LeafGreen and Ruby, Sapphire, or Emerald, the trainer must first help Celio fix the inter-regional trading machine on One Island after defeating the Elite Four .
5. In Generation IV, Pokémon can again be trade between games via the Wireless Club on the second floor of any Pokémon Center. In addition, Pokémon can now be traded over wi-fi at the Global Trading Station in Jubilife City with other Gen. IV games all over the world.
6. In Generation V, it is possible to trade via the C-Gear from almost anywhere in Unova, not just in the Pokémon Centers like previous generations. The only restriction is that nothing with an HM can be traded.
7. In Generation VI, the Player Search System replaces the C-Gear. Allowing to make trades with just a tap of a button.
8. There is a feature newly added in Generation VI called Wonder Trade, which has one send a Pokémon out and almost immediately receive a random Pokémon from a random player.
9. In Generation VII, it is possible to trade via the Festival Plaza or Quick Link. The Festival Plaza is for using the GTS and Wonder Trading. Quick Link is for quickly trading with people nearby.

Notes

• Traded Pokémon cannot be given a new nickname, including Pokémon named in a different language.
• Pokemon that were traded from a different game receive a 30% experience boost on any Experience Points they receive.
• In the anime, why Trainers simply don't exchange Poké Balls by hand is never explained. However, it's possible that putting them through the trading machines reprograms the Poké Balls to allow them to be teleported back to the correct Professor's lab when the Trainer's party exceeds six Pokémon.