A downhole monitoring method comprising: - setting at least one barrier in a cased borehole, the at least one barrier including a column of flowable sealing material, such as cement, having a height of at least 2m, such that pressure and fluid communication are resisted across the borehole thus separating the borehole into a lower section below the at least one barrier and an upper section above the at least one barrier; - bonding said column of flowable sealing material to a portion of formation which defines a portion of the borehole; at least a portion of the lower section being cased with casing, thus defining an annulus between the surrounding formation and the casing; wherein there is provided an assembly in the lower section, including: a perforating device; a control mechanism to control the perforating device, and comprising a wireless communication device configured to receive a wireless control signal for activating the perforating device; a pressure sensor; - at any time, sending the wireless control signal to the wireless communication device to activate the perforating device, the wireless control signal transmitted in at least one of the following forms: electromagnetic, acoustic, inductively coupled tubulars and coded pressure pulsing; - after the at least one barrier is set, activating the perforating device, in order to create at least one perforation through the casing; - after the perforating device has been activated: (i) monitoring the pressure in the lower section below the at least one barrier using the pressure sensor; and, (ii) sending a wireless data signal including pressure data from below the at least one barrier to above the at least one barrier, using at least one of electromagnetic communication and acoustic communication and after step (ii) (iii) assessing whether the lower section is, or to what extent, isolated from the upper section. - setting at least one barrier in a cased borehole, the at least one barrier including a column of flowable sealing material, such as cement, having a height of at least 2m, such that pressure and fluid communication are resisted across the borehole thus separating the borehole into a lower section below the at least one barrier and an upper section above the at least one barrier; - bonding said column of flowable sealing material to a portion of formation which defines a portion of the borehole; at least a portion of the lower section being cased with casing, thus defining an annulus between the surrounding formation and the casing; wherein there is provided an assembly in the lower section, including: a perforating device; a control mechanism to control the perforating device, and comprising a wireless communication device configured to receive a wireless control signal for activating the perforating device; a pressure sensor; a perforating device; a control mechanism to control the perforating device, and comprising a wireless communication device configured to receive a wireless control signal for activating the perforating device; a pressure sensor; - at any time, sending the wireless control signal to the wireless communication device to activate the perforating device, the wireless control signal transmitted in at least one of the following forms: electromagnetic, acoustic, inductively coupled tubulars and coded pressure pulsing; - after the at least one barrier is set, activating the perforating device, in order to create at least one perforation through the casing; - after the perforating device has been activated: (i) monitoring the pressure in the lower section below the at least one barrier using the pressure sensor; and, (ii) sending a wireless data signal including pressure data from below the at least one barrier to above the at least one barrier, using at least one of electromagnetic communication and acoustic communication and after step (ii) (iii) assessing whether the lower section is, or to what extent, isolated from the upper section. (i) monitoring the pressure in the lower section below the at least one barrier using the pressure sensor; and, (ii) sending a wireless data signal including pressure data from below the at least one barrier to above the at least one barrier, using at least one of electromagnetic communication and acoustic communication and after step (ii) (iii) assessing whether the lower section is, or to what extent, isolated from the upper section.