Anesthetic drugs significantly affect the brain physiology, inducing unconsciousness and eventually imbalance between delivery and consumption of oxygen with catastrophic consequences. In addition, the surgical insult, peripheral or direct in the case of brain surgery, triggers a potent neuroinflammatory reaction that may lead to postoperative neurocognitive disorders. Monitoring the cerebral function and the cerebral metabolism constitutes a mandatory task to titrate and guide dosing schemes of anesthetic drugs and to optimize cerebral perfusion. This chapter provides an overview of the rationale behind perioperative brain monitoring, serving as a key introduction to the subsequent chapters of this textbook.