When you get the Master Ball in Team Aqua's/Magma's lair, do not use it on Kyogre/Groudon. Latios and Latias will eventually appear in your game, and they are extremely difficult to catch. They will run from route to route.

Just after you finish the game, on television there will be a report about a one-of-an-kind Pokemon that is now loose in Hoenn. It will show up all over Hoenn in the grassy areas. You will only have one chance to catch it. It rarely appears. It depends on which game you are playing; Latios for Pokemon Ruby and Latias for Pokemon Sapphire.

When going after Latios and Latias, always use Repels so you are not interrupted by other Pokemon.