A trade is a process in which a Pokémon Trainer sends one of his or her Pokémon to another Trainer in exchange for one of the other Trainer's Pokémon. In every Pokémon game, trading is necessary to collect all Pokémon.
Starting from Generation II, items can be held by Pokémon, allowing indirect trading of holdable items. However, in Generation IV, the 
Griseous Orb cannot be traded, as it will automatically be put back in the Bag when a player with Origin Forme Giratina in their party enters the Union Room or Wi-Fi Room. Certain items, when held by the correct Pokémon, will cause that Pokémon to evolve when traded to another player. Mail may also be held to send a message.
Until Black 2 and White 2, all trading animations games used the standard 
Poké Ball, regardless of the types of Balls the traded Pokémon were actually caught in. This is corrected in Black 2 and White 2, in which trade animations show the correct Poké Ball the Pokémon was caught in.
Trading is necessary in order to collect every Pokémon in the Pokédex, as some Pokémon can only be found in certain versions. For example, because Meowth cannot be found in Pokémon Red and Blue, the player must trade with someone who has obtained one from Blue, in which Meowth is readily found in the wild. Some Pokémon only evolve after being traded.
Traded Pokémon gain 1.5× the normal experience after a Pokémon battle. Pokémon traded from a game in another language will gain 1.7× experience.
Trading may also be used to transfer limited and rare items in games from Generation II onward, such as 
Master Balls or 
Soul Dews, from one game to another by giving a Pokémon an item.
In Generation VI, Trainers receive Poké Miles for every trade made with another player.
A Pokémon with an original Trainer different from its current Trainer is referred to as an outsider Pokémon, and will only obey a Trainer with the sufficient number of Gym Badges. The friendship of a Pokémon is set to 70 when it is traded from one game to another, unless returned to its original Trainer. In addition, a traded Pokémon's nickname cannot be changed by anyone but the original Trainer, even if it hasn't been given a nickname.
Traded Pokémon are identified by the Pokémon's Original Trainer name and a five-digit ID Number. Starting in Generation III, even if two games have the same name and ID number, each Trainer also has a secret ID number. The odds of two Trainers having the same secret ID numbers is 1/65536 or approximately 0.002%, making it extremely unlikely that an outsider Pokémon will be treated as a regular Pokémon on a different cartridge.