The immediate and successful management of primary, secondary, and mixed acid-base disorders, urgent or emergency, concerns urgent and intensive care medicine and beyond. It is a particularly complex but also frequent, almost daily, medical problem that affects many specialties such as pulmonology, nephrology, pathology, emergency medicine and intensive care, cardiology, not only in terms of urgent incidents but also in terms of outpatient care. The purpose of this edition is to highlight the complexity of this issue, to analyze it in a clear and simple way, and to lead to a rapid understanding of the basic pathophysiological phenomena and disorders of acid-base balance.