Abstract: The invention relates to sterilisation systems suitable for clinical use, e.g. on the human body, medical apparatuses, or hospital bed spaces. In particular, the invention provides a sterilisation apparatus comprising: a sterilisation device having a hydroxyl radical generating region and an outlet for directing generated hydroxyl radicals out of the hydroxyl radical generating region towards a region to be sterilised; and a steam supply connected to deliver dry steam to the sterilisation device, wherein the sterilisation device is configured to generate a high impedance at the hydroxyl radical generating region when dry steam, radiofrequency (RF) and/or microwave frequency electromagnetic (EM) energy, and gas are delivered to the hydroxyl generating region, thereby to create a plasma of the gas which forms an ionisation discharge for generating hydroxyl radicals from the dry steam for delivery out of the hydroxyl generating region.