In this paper we consider the cryptanalysis of the block cipher SMS4. The cipher has received much recent attention due its simplicity and prominence (it is used in wireless networks in China) and a range of differential attacks break up to 21 of the 32 rounds used in SMS4. Here we consider the application of linear cryptanalysis to the cipher and we demonstrate a simple attack on 22 rounds of SMS4. We also consider some advanced linear cryptanalytic techniques which, under the best conditions for the cryptanalyst, might (just) extend to 23 rounds.