This guideline covers preventing and managing inadvertent hypothermia in people aged 18 and over having surgery. It offers advice on assessing patients’ risk of hypothermia, measuring and monitoring temperature, and devices for keeping patients warm before, during and after surgery. In December 2016, we reviewed the evidence on measuring temperature, warming patients before induction of anaesthesia and warming patients after induction of anaesthesia. We changed and added some recommendations in sections 1.1, 1.2 and 1.3.