Stress is a part of life. All change leads, in all of us, to an adaptation reaction, the objective of which is to maintain homeostasis; this is a non-specific phenomenon, which applies to all animal species. A small cat, when it sees a big dog, will react: its heart will start beating faster, which means it will take in more oxygen and produce more energy, which will give it a better chance of survival. It will quickly choose a strategy: either confront the dog, or retreat (‘fight or flight’). It will generally choose to run away, and once the danger is gone, its body will slowly calm down and recover its energy. This is an example of well-managed stress, where energy expenditure was adjusted for an immediate, ideally suited action. But if that little cat had decided to confront the dog, or taken too long to decide, its response would not have been appropriate; its energy expenditure would have been too large or lasted for too long, and it would have led to exhaustion, eliminating any chances of survival. Stress response therefore depends on two factors: the stress itself, on the one hand, and the stressed subject, on the other hand. Once the brain has interpreted a situation as stressful, the stress response is triggered.