When an animal is under stress, many physiological modifications take place in order to ‘prepare’ its body to cope with the challenge. Physiological and behavioural responses have the goal of addressing the challenge and therefore they are adaptive, increasing survival chances and fitness. The chapter looks at the physiology of stress, impact of stress during the developmental period, what can be considered a stress stimulus?, stress as a risk factor for diseases and behavioural problems, stress and emotions, effect of stress on learning and memory, stress-related psychogenic problems, management of stressed animals during veterinary visits.