Stress is defined as the reaction of an organism triggered by a treat that requires the animal to adapt. The response to stress can have a general or local effect, depending on intensity and duration of the stimulus. The most important stressors are: temperature extremes, transportation, inadequate housing, social interactions, nutritional stress, animal handling and diseases. Stress reaction is regulated by the central nervous system, endocrinological and immune system. The hypothalamic-pituitary-adrenal axis plays the main role in the stress response, by secretion of glucocorticoids, which are known as stress hormones. They induce changes in organ systems of an individual, the animal's energy and resources are used to cope with the noctious stimulus. Promoting survival functions leads to inhibition of non-essential functions, such as growth and development, reproduction, productivity and disease resistance. It has been shown that acute stress stimulates immunity, while chronic stress supresses immune functions. Because of the inhibiting effects of stress on the immune system, animals suffering from stress have increased susceptibility to diseases such as: transport fever, mastitis, ketosis, enterotoxemias, salmonellosis and other enteropathogens, and porcine stress syndrome. Temperature extremes have a negative effect on feed intake, resulting in reduced growth and production, especially milk production. Harsh animal handling and immobilization are strong stress stimuli, particularly in young animals, which can be avoided in order to reduce production of lesser quality meat. Optimal reproduction depends on homeostasis, in other words on proper housing and animal handling, quality feeding and stable hierarchical relations. Stress reduces fertility, and consequently the production, insemination as well as possible treatment costs rise. In conclusion, the stress response inhibits non-essential functions, such as reproduction, growth and digestion, in favour of those functions that are vital for survival.