A method of pressure testing a casing system (12) comprising deploying an apparatus (110) with a container (68) formed from drill pipe or production tubing, on a string of drill pipe or production tubing into a well, such that a port (61) thereof is provided in an isolated section of the well. The isolated section of the well has a higher pressure than the pressure of a gas sealed in the container (68). The port (61) is opened and pressure in the isolated section is monitored to assess its integrity. Communication with the apparatus is achieved by wireless means sending pressure data, control signals for controlling the port and/or other data or commands.