HBO has officially renewed "Game of Thrones" for season 4.

Everyone working on the hit fantasy drama has been operating under the assumption that there would be a fourth season, as producers David Benioff and D.B. Weiss have elected to split George R.R. Martin's third "A Song of Ice and Fire" book, "A Storm of Swords," over two seasons, with this being the first. But the contracts for a fourth season weren't finalized until today.

The show returned to series-high ratings on Sunday night.

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Alan Sepinwall has been reviewing television since the mid-'90s, first for Tony Soprano's hometown paper, The Star-Ledger, and now for HitFix. His new book, "The Revolution Was Televised," about the last 15 years of TV drama, is for sale at Amazon. He can be reached at sepinwall@hitfix.com