A shear valve (10) comprises a casing having a first part (12) for positioning on one side of a pipe and a second part (14) engageable with the first part (12) for positioning on the other side of the pipe. A blade (26) is disposed substantially within the first part and is actuable across the pipe towards the second part for cutting the pipe. The blade has a support region for supporting the periphery of the pipe as the pipe is being cut.