Heat stress is one of the most important environmental stressors that reduce productivity in animal breeding. The productivity of farm animals is significantly reduced due to temperature increases. As a result, significant economic losses occur during the summer months. Temperature humidity index developed according to air temperature and relative humidity relation is a widely used method for determining the effect of heat stress for domesticated animals. The ruminants entering the heat stress have lower feed consumption and consequently lower yields. This review was conducted to determine the effects of negative stressors on ruminants, to show how to reduce the effects of these factors, and to determine what the physiological changes due to temperature stress are.