Provided are imidazol [1,5-f] [1,2.4]triazinyl, imidazol [1,2,4]triazinyl, and[1,2,4]triazolo [4,3-f] [1,2,4] triazinyl nucleosides of formula I, nucleosidephosphates andprodrugs thereof, wherein X1 or X2 is independently C-R10 or N and wherein atleast oneof X1 and X2 is N. The compounds, compositions, and methods provided may beusefulfor the treatment of Flaviviridae virus infections, particularly hepatitis Cinfections.(see formula I)