Acid-base disturbances are common, and can have a significant impact on patient morbidity and mortality if unrecognized or if treated inappropriately. Blood gas analysiscan be used to assess a patient’s acid-base status and also the oxygenation of the blood. This chapter discusses buffering, blood gas analysis, simple acid-base disorders, responses to acid-base disturbances, evaluation of samples, blood gas analysis in respiratory patients, effects of blood gas disturbances on other analytes and mixed acid base disturbances. The chapter includes case examples.