Like Pokémon Diamond & Pearl, the regional Pokédex and the areas around it are only the tip of the Sinnoh Iceberg. Like the past games, you need to complete the Sinnoh Dex in order to get access to the National Dex. The National Dex is a complete Pokédex capable of containing all the Pokémon from Bulbasaur to Arceus.

The method of getting the National Pokédex is to complete the Sinnoh Dex. Like in Diamond & Pearl, this is done by merely seeing the Pokémon as opposed to capturing them. This makes it easier than it was in the 3rd generation games. However, Pokémon Platinum has a further 59 Pokémon in the Pokédex and you need to see all 210 of them as opposed to seeing the first 150 in Diamond & Pearl. For a list of all Sinnoh Dex Pokémon, click here. This is a very daunting task, but if you battle every trainer you see, you should be able to fill up your Pokédex as you travel through Sinnoh with very little difficulty.

That said, there are several Pokémon that people will miss due to the fact they do not get used by trainers in the game or you do not automatically get their Pokédex entry when you progress through the story like Dialga & Palkia do. As such, here is a list of the more commonly missed Pokémon

The first one that you likely will not see is Unown. No trainers have this alphabet Pokémon, and the one place that it inhabits is seldom visited. Although Defog is placed at the bottom of Solaceon Ruins, Unown does not appear to activate until you get to the bottom. As such, people who escape rope out of the ruins will not get to battle this Pokémon

Manaphy is an event Pokémon that is only obtainable in conjunction with the two Ranger games. Due to this, many people will not have it and so you cannot battle it. In order for it to be seen in your Pokédex, go to Mr. Backlot's Mansion in Route 212 and into his main room where he is. On the desk there should be a book and if you examine this book, Manaphy will be seen and added as such to your Pokédex

Rotom is not owned by any trainers within the game. You do however walk past the area it inhabits whenever you leave Eterna Forest. Rotom resides in one of the Televisions within the Old Chateau. It is only available from 8pm in the day and there is only one so you have to be careful when fighting it if you wish to keep it.

After completing the Dex, go to Professor Rowan's laboratory. He will congratulate you on completing the Sinnoh Pokédex just as Professor Oak comes in to the building. He will notify you that there are many more Pokémon in the Sinnoh Pokédex and you will be given the National Pokédex. It is as simple as that.

A variety of areas and functions become available after you obtain the National Dex including Swarms, the PokéRadar, Pal Park and the Battle Zone in the North East of Sinnoh