A Pokédex(Romanized Japanese: Pokémon Zukan, Brazilian: PokéAgenda) is an invaluable tool to a trainer in the Pokémon world. It gives information about (almost) all Pokémon in the world that are contained in its database. In the games, it lists the number of Pokémon seen and caught. In the anime, simply pointing it at a target Pokémon will give someone the Pokédex entry for that Pokémon. Pokédexes in both the games and anime are not given to all trainers, but given to trainers of exceptional potential and skill. In both the games and the anime, Ash and Gary both got Pokédexes from Prof. Oak, although via different circumstances. Ash was notorious for using his Pokédex in situations where he had no idea what was going on, and was just as notorious for always being behind Gary in Pokédex entries.

Pokédex entries are typically only two- or three-sentence bits of information about the Pokémon referred to. In some events it gives background information on the habitat or activities of this Pokémon in the wild; at other times it gives other bits of information on the Pokémon's history or anatomy. A typical entry would be one of Bulbasaur, which states:

"A strange seed was planted on its back at birth. The plant sprouts and grows with this Pokémon. "

To date, there have been four different versions of the PokéDex:

Kanto (Generation I)

The first version of the PokéDex recorded data on the first 151 Pokémon, focusing on Pokémon mostly native to Kanto. It listed them in number sequence, and it recorded data on height, weight, number, species, cry, and nesting area. In Pokémon Yellow, the option to print PokéDex entries using the Gameboy Printer was added.

In the anime, when Misty got Togepi, Professor Oak updated this model with new software that included data on a handful of Johto Pokémon.

Johto

The second PokéDex had many improvements. Data on 251 species of Pokémon could be recorded. Pokémon could be listed in the Kanto (or National) order, a new order focusing on Johto, or by alpabetical organization. A new search option allowed users to find specific Pokémon by Type. Entries were expanded to include a Pokémon's footprint (if it had feet). Also, the design of this PokéDex is markedly similar to that of Nintedo's GameBoy, GameBoy Pocket, and GameBoy Color.

Hoenn

The third PokéDex could record data on 202 species of Pokémon native to the Hoenn region. An upgrade called the NationalDex was unlocked if a trade with Kanto was made, or if the Elite Four were defeated in Pokémon Emerald, which would allow 386 species to be recorded. Pokémon could also be listed by weight and size. The search option was expanded to include the first letter of a name, as well as color of the Pokémon. The most interesting addition was size charts that compared a 10-year old child to inidividual Pokémon in terms of height and weight. This allowed trainers to get a better idea of how large individual Pokémon were, which often conflicted with depictions in the anime. The print option was removed, however.

Kanto (Generation III)

The fourth PokéDex could record data on 151 species of Pokémon native to the Kanto region. An upgrade called the NationalDex was unlocked if the Elite Four were defeated, which would allow 386 species to be recorded. In addition to the improvements made in the Hoenn version, Pokémon were all organized into habitat categories (Grassland, Forest, etc.) Similar Pokémon were typically listed together on the same pages (such as Jynx and Mr. Mime, for instance). Nest display was also expanded to include the Sevii Islands.