In the transition between the games Gold & Silver and Ruby & Sapphire, many trainers were disappointed and outraged at the lack of the capability for transferring their Pokémon. However in Diamond & Pearl, they have rectified this and allow you to trade your Pokémon from Ruby, Sapphire, Emerald, FireRed or LeafGreen over to Diamond & Pearl. However since the DS cannot physically link to the GBAs, you have to do this a different way than expected. First you need to have obtained the National Dex and accessed Pal Park. Next, make sure the GBA game plugged into the GBA slot in the DS and Diamond or Pearl into the DS Slot. Then when you load up your DS, select Diamond or Pearl.
3rd Gen Requirements

There are no limits on the 3rd Generation Games, you just have to have at least 7 Pokémon on your entire game

4th Gen Requirements

To allow for this transfer, you need to have beaten the Elite Four and have the National Pokédex. Once you have done this, Professor Oak will invite you to Pal Park. Go there and save, Now, if you have the GBA game in your DS, you should have the option to transfer Pokémon

Transfer Conditions

There are a number of conditions of the transfer that you need to be aware of. Firstly, the transfer is one way. Your Pokémon will be gone from the 3rd Gen games and will not be able to put them back on the game.
Go to the menu on the game and you will see an option for transfer. Go into it and it'll ask you to select 6 Pokémon. Select these six Pokémon and it will say that they are in Pal Park

Now go into your game and go to Pal Park. In there, you will find your Pokémon in the tall grass or Waters all along Pal Park, dependant on their type. In the Park, you have Park Balls which have a 100% capture rate. You don't battle the Pokémon so you cannot accidentally knock it out. You are also capable of leaving the park at any time. You are just unable to transfer any more Pokémon until you have re-captured all of them
This method is the only way you have to get some Pokémon such as Deoxys so you will use it as time goes on.

Everything about the Pokémon stays the same; Nicknames, Item Attached, IDs, Attacks...everything stays exactly the same as before. So there is no need to worry about losing items, and this is the perfect way to get old items onto your Diamond & Pearl games

There is one final snag, Pal Park is region blocked, if you have a Japanese GBA game, it will not work on a US Diamond and so forth for every possible combination.