Acid-base balance is an vital invasive investigative procedure of the respiratory assessment. Any disorders in the normal homeostatic acid-base balance will lead to altered functioning of the body. The two major disorders are, either the acidotic state or the alkalotic state as reflected by the pH of the arterial blood. PaCO2 reflects the functioning of the lungs i.e either hypoventilation or hyperventilation. On the other hand, HCO3 reflects the function of the kidney i.e either absorption or removal of bicarbonate ions to normalize the pH. Either the lung or the kidney will compensate to correct the acid-base disorder. The physical therapist should be able to assess and interpret the disorders of the acid-base balance accurately and guide his treatment based on the findings of arterial blood gases. This book includes the explanation of the cause of these acid-base disorders, their assessment and the correction of the same.