A method to test or monitor the integrity of a cement barrier, comprising providing an assembly therebelow including: a perforating device; a control mechanism to control the perforating device, a pressure sensor, a wireless communication device and a pressure sensor. The perforating device is activated to perforate casing below the barrier and a pressure test may be conducted. The creation of the perforation(s) below the barrier allows an assessment of the integrity of the barrier across its entire width, and especially its bond to the formation, rather than only a central portion of the barrier. Electromagnetic or acoustic wireless signals are used to retrieve data from below the barrier.