ABG is “gold standard\" to analyze ventilation and perfusion. Respiratory acidosis occurs when an increase in PaCO2 causes a decrease in pH. Respiratory alkalosis occurs when a decreases in PaCO2 causes an increase in pH. Metabolic acidosis occurs when there is either an excess of lactic acid or a deficiency in the buffering capacity of the blood, causing a decrease in pH. It is reflected in an increased base deficit, also termed a decreased base excess. This chapter describes steps in interpretation of ABG and approach to various acid-base disorders. Permissive hypercapnia was a novel strategy used in mechanically ventilated babies to reduce the incidence of chronic lung disease. Current understanding of this strategy dictates that any pCO2 more than 55 and a pH less than 7.2 is to be treated with great caution with regards to neurodevelopmental outcome. This concept of permissive hypercapnia should ideally not be used in the acute phase of lung disease (FiO2 more than 0.6). The author feels that improvement in pH usually parallels improvement in respiratory status and work of breathing. Respect pH and babysit CO2.