In tissue treatment apparatus for skin resurfacing having a handheld treatment instrument with electrodes connected to a radio frequency generator, a plasma jet is produced by ionising gas passing through a gas conduit in the instrument, the plasma emerging from a nozzle at an end of the conduit. Incorporated in the instrument is an optical detector that receives radiation emitted by the plasma directly from within the conduit and produces output signals which are processed to indicate a fault condition in the absence of radiation within a predetermined interval after the commencement of delivery of RF energy to the instrument or if the radiation is not of an approximately constant level during the delivery of RF energy.