A method of pressure testing a casing system (12a, 12b) of a well system, the method comprising: - providing an apparatus comprising a communication device, and a container with a volume of at least twenty litres, the container having a port to allow for fluid communication between an inside and an outside of the container; - closing the port such that the inside of the container has a container pressure; - deploying the apparatus into the well system on one of production tubing and drill pipe, said one of production tubing and drill pipe comprising joints of 3m to 14m, and nominal external diameter from 2 7/8" diameter and at most 7" diameter; - isolating a section of the well system to provide an isolated section (13, 113), the isolated section including the port of the container; the isolated section, outside of the container, having an isolated section pressure which is higher than the container pressure; - reducing the pressure in the isolated section outside of the container, by opening the port to allow fluid communication between the inside of the container and an outside of the container; and, - whilst maintaining isolation of the isolated section, monitor the pressure in the isolated section; - using the monitored pressure to assess the integrity of the isolated section; - communicating between the communication device in the isolated section and above the isolated section by using, at least in part, a wireless signal transmitted in at least one of the following forms: electromagnetic, acoustic, inductively coupled tubulars and coded pressure pulsing; - wherein the wireless signal comprises at least one of (i) pressure data and (ii) control signals for controlling fluid flow through the port. - providing an apparatus comprising a communication device, and a container with a volume of at least twenty litres, the container having a port to allow for fluid communication between an inside and an outside of the container; - closing the port such that the inside of the container has a container pressure; - deploying the apparatus into the well system on one of production tubing and drill pipe, said one of production tubing and drill pipe comprising joints of 3m to 14m, and nominal external diameter from 2 7/8" diameter and at most 7" diameter; - isolating a section of the well system to provide an isolated section (13, 113), the isolated section including the port of the container; the isolated section, outside of the container, having an isolated section pressure which is higher than the container pressure; - reducing the pressure in the isolated section outside of the container, by opening the port to allow fluid communication between the inside of the container and an outside of the container; and, - whilst maintaining isolation of the isolated section, monitor the pressure in the isolated section; - using the monitored pressure to assess the integrity of the isolated section; - communicating between the communication device in the isolated section and above the isolated section by using, at least in part, a wireless signal transmitted in at least one of the following forms: electromagnetic, acoustic, inductively coupled tubulars and coded pressure pulsing; - wherein the wireless signal comprises at least one of (i) pressure data and (ii) control signals for controlling fluid flow through the port.