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Stress is a major biological mechanism that aims to optimize the way an organism copes with a constantly changing environment. Stress is a physiological function with inputs (interoceptive and exteroceptive perception), brain integrative centres (valence and control evaluation, etc.), outputs (stress mediators: the hypothalamic-pituitary-adrenal [HPA] axis and the autonomic nervous system with its sympathetic and parasympathetic branchs), and regulatory systems that adjusts the activation level of each stress mediator as well as the overall stress level to the demand. Stress helps to mobilize an organism's resources to cope efficiently with a perceived challenge. Stress response not only adapts itself to the present threat but also memorizes it via mechanisms such as memorization (conditioning) and epigenetic modulation in order to optimize its future response to a similar stressor. Therefore, the history of the individual shapes stress over time and increases the intrinsic interindividual variability of stress responses. Lastly, stress integrates the biological cost of its response by triggering recovery mechanisms in a diachronic way through sleep, parasympathetic activation, and tissue regeneration. The aim of stress is therefore to increase an individual's chances of survival and to allow adaptative mechanisms to develop. Unfortunately, stress can become pathogenic when its regulation is defective, if it is either excessive or insufficient. This explains a number of stress-related pathologies.