The fifth StarCraft II League season has arrived! At this time, Season 4 milestones have been awarded, and bonus pools and standings have been reset. Hidden skill ratings used for matchmaking and league placement have carried over from the previous season, so players who have completed placement matches in a previous season will only need to play one new placement match to kick off the new season.

Source:
http://us.battle.net/sc2/en/blog/4103100...