Although it is generally assumed that stress is detrimental to the well-being of poultry, there are positive aspects which are often overlooked. In the context of evolution, stressors such as infectious agents, climatic extremes, and domestication processes play important roles in inter-population and intra-population genetic variability. Pathological, immunological and behavioural studies in both avian and mammalian species indicate that too much or too little stress is undesirable, and that a certain degree of stress is essential for maintaining normal biological functions. Prior stressful experiences, particularly during the neonatal stage, may have long-term benefits in helping an individual to cope with subsequent stressors. Reliability of stress indicators and their variability at both population and individual levels is a major factor in attempting to quantify ‘optimum stress’.