Abstract: A plug for creating a pressure barrier within an oil or gas well comprises a plug body (5) and a metal primary seal element (7) having a frusto-conical seal face (7A) adapted to engage a corresponding seal surface of a profiled well conduit to create a metal-to-metal seal having a relatively large contact area. The primary seal is energised by pressure applied above the plug, prior to the plug been set by a running tool which operates an actuating member (5) to displace locking keys (4) radially outwards so as to engage corresponding formations in the well profile and lock the plug against axial movement within the well conduit. The plug may be retrieved using a pulling tool and re-used, and includes a secondary elastomeric or polymeric seal (6).