For any serious Pokemon trainer, Pokemon Bank is a solid investment. This handy app connects with Sun and Moon and any of the 6th generation games (X, Y, Omega Ruby and Alpha Sapphire) and lets you transfer and store up to 3,000 Pokemon online.
Pokemon Bank is available to download from the Nintendo 3DS eShop for free, although you need to pay a nominal fee (4.99 USD) to use its services for a year. If you don't renew after the year's over, you can still withdraw your Pokemon, but depositing Pokemon will become impossible.
Additionally, Pokemon Bank comes with a companion app called Poke Transporter that can import Pokemon from the 5th generation games (Black, White, Black 2 or White 2) or Virtual Console games (Red, Blue and Yellow) to Pokemon Bank, making them ready for use in the recent games.
The Small Print
Compatibility with Sun and Moon and the Virtual Console games will be available when a free update to Pokemon Bank and Poke Transporter goes live during January 2017.
Pokemon sent to Sun and Moon or imported from the Virtual Console games can only be transferred to Sun and Moon, not any of the 6th generation games.
Hold On a Moment!
While Pokemon Bank is fabulous for storing your massive Pokemon collection and transferring between multiple games, keep in mind that Pokemon will not keep their hold items. Therefore you'll need to rebuild your item collection in your new games.