The three parts of the respiratory control system are: sensors, central control and respiratory muscles. The different disorders of ventilation include hypoventilation syndromes and hyperventilation syndromes. The three main types of hypoventilation syndromes are: obesity hypoventilation syndrome, respiratory neuromuscular disorders and primary alveolar hypoventilation. Alveolar hyperventilation exists when PaCO2 decreases below 4.5 kPa. It is not the same as hyperpnoea which is increased minute ventilation with a normal PaCO2. Detailed history and clinical examination along with knowledge of coexistent disorders can provide clues to the cause of hyperventilation. An elevated pH is suggestive of a primary respiratory alkalosis; a low pH will show a metabolic acidosis. Widened A-a gradient suggests presence of a primary pulmonary disorder. Low bicarbonate suggests a chronic nature of the disorder and implies an organic cause. Transcutaneous PCO2 or arterial PCO2 during sleep studies can help to exclude psychogenic hyperventilation.