A system for isolating a wellbore having a fluid flow passage extending from surface to a subterranean location has a first, downhole, valve and a second, uphole, valve. The downhole valve is located in the wellbore at a first subterranean location and the uphole valve is located in the wellbore at a second subterranean location spaced from the first valve. Downhole valve is operable between a first configuration permitting access through the flow passage and a second, well isolation, configuration isolating the flow passage below the valve. Uphole valve is operable between a first configuration permitting access therethrough and a second, isolation, configuration providing an isolated or isolatable volume between the first and second valves. A pressure sensor disposed in the isolated volume between the downhole valve and the uphole valve and, in use, permits a pressure in the isolated volume to be measured and/or communicated to a remote location.