Respiratory failure occurs due mainly either to lung failure resulting in hypoxaemia or pump failure resulting in alveolar hypoventilation and hypercapnia. Hypercapnic respiratory failure may be the result of mechanical defects, central nervous system depression, imbalance of energy demands and supplies and/or adaptation of central controllers.Hypercapnic respiratory failure may occur either acutely, insidiously or acutely upon chronic carbon dioxide retention. In all these conditions, pathophysiologically, the common denominator is reduced alveolar ventilation for a given carbon dioxide production.Acute hypercapnic respiratory failure is usually caused by defects in the central nervous system, impairment of neuromuscular transmission, mechanical defect of the ribcage and fatigue of the respiratory muscles.The pathophysiological mechanisms responsible for chronic carbon dioxide retention are not yet clear. The most attractive hypothesis for this disorder is the theory of “natural wisdom”. Patients facing a load have two options, either to push hard in order to maintain normal arterial carbon dioxide and oxygen tensions at the cost of eventually becoming fatigued and exhausted or to breathe at a lower minute ventilation, avoiding dyspnoea, fatigue and exhaustion but at the expense of reduced alveolar ventilation. Based on most recent work, the favoured hypothesis is that a threshold inspiratory load may exist, which, when exceeded, results in injury to the muscles and, consequently, an adaptive response is elicited to prevent and/or reduce this damage. This consists of cytokine production, which, in turn, modulates the respiratory controllers, either directly through the blood or probably the small afferents orviathe hypothalamic-pituitary-adrenal axis. Modulation of the pattern of breathing, however, ultimately results in alveolar hypoventilation and carbon dioxide retention.