This chapter describes various fundamentals of acid-base metabolism and provides a structured guide to diagnosis and management of various acid-base disorders, with special emphasis on those seen in perioperative period. Arterial blood gas monitoring is used to monitor acid-base balance, and also to measure the adequacy of oxygenation and ventilation. It is essential for management of critically ill patients in the operation theater as well as in intensive care unit (ICU). Measurement of arterial blood gas requires procurement of arterial blood sample from the patient. A stepwise approach, using data from blood gas analysis and the clinical context of presentation, would allow accurate identification of the acid base disorder(s) and its underlying etiology. Obtaining arterial blood for analysis, indications for arterial blood gas monitoring, components measured with arterial blood gas analysis, basic concepts of acid-base homeostasis, simple acid-base disorders, principles for further characterization of acid-base disorders, etc. are some other important topics also covered in this chapter.