Game of Thrones was renewed for two more seasons on Tuesday, two days after its season four premiere. The fourth season premiere drew 6.6 million viewers -- HBO's largest audience since the final episode of The Sopranos in 2007. Sunday night's Game of Thrones premiere was watched by so many viewers that the episode crashed on HBO Go, the network's online streaming video service. Game of Thrones viewership has steadily increased -- the season three premiere drew 4.4. million viewers in comparison -- and the network obviously expects the series' popularity to continue. Seasons five and six are now officially a go, although their exact content has not yet been revealed. The current fourth season roughly covers the second half of the third A Song of Ice and Fire book, A Storm of Swords. The next book in the series is A Feast for Crows, which is some 500-odd pages shorter than A Storm of Swords. It remains to be seen if A Feast for Crows will be split between the two upcoming seasons, or if season six of the show will cover the series' fifth book, A Dance with Dragons. Game of Thrones airs at 9 p.m. ET Sundays on HBO.