With increasing global temperatures, the risk of heat stress for farm animals is also growing. As pigs are not able to sweat and in livestock houses they generally do not have the opportunity to wet themselves, an ambient temperature above the upper critical temperature (UCT) leads to reduced welfare and performance. This UCT varies per pig category (including weight) and is considerably lower for lactating sows than for newborn piglets. At high humidity even lower temperatures can lead to heat stress, i.e. above a calculated temperature-humidity-index (THI) above 84 for finishers. In case of upcoming heat stress the pig reacts in different physiological and behavioural ways to maintain homeothermia. The pig becomes less active, reduces feed intake, adapts its lying behaviour and increases breathing frequency. If these measures are not sufficient the core body temperature will rise and ultimately lead to death. The scientific knowledge in this review aims to support welfare inspectors in the field of climate control on pig farms, in particular at high temperatures. Useful indicators for heat stress can be environment based or animal based. Environment based indicators are the above mentioned room temperature and relative humidity, combined as THI. But also the absence of air cooling or pig cooling facilities increase the risk of heat stress. Animal based indicators are animal and pen fouling, skin and body temperature, respiration (breathing frequency, panting, open mouth), lying pattern, feed intake and water intake. Room temperature, relative humidity, panting and pig fouling are recommended in this review as the most useful and proven indicators for heat stress. Measures to prevent heat stress can preferably be combined with the weather forecast to prevent heat accumulation in pig buildings. The main measure is to regularly check indoor temperature and relative humidity and to use a table on corresponding THI values to assess the heat burden of animals and to start with countermeasures before the threshold is reached. The feed ration can be lowered to prevent mortality and left overs in the trough. To prevent arousal in the house, the feeding time could be postponed to cooler period, taking into account the legal requirements for feeding. If possible, veterinary treatments should be postponed. Drinkers should be checked (flow rate, fresh, clean, cool) and ventilation rate maximized. Incoming air can be cooled by water evaporation: water spraying (fog/mist) in the air inlet. Air speed can be increased by inside circulation of air and at very high temperatures this could be combined by wetting the pigs. Direct sunshine through the windows can be prevented by insulation or use of white chalk. Chalk or moisten the roof can also keep the roof somewhat cooler. As pigs like to maximize heat loss by lying laterally without contact with penmates the caretaker shall try to maximize the space allocation per pig.