OBJECTIVE: To describe the main physiopathologic mechanism of the respiratory failure in the child, as well as to discuss some aspects of the differential diagnosis and treatment. SOURCE OF DATA: The main national and international textbooks and articles about respiratory failure in the child were used as sources of data for this research. RESULTS: Respiratory failure is defined as the incapacity to maintain a paO(2) over 50 mmHg associated or not to a paCO(2) over 50 mmHg in children breathing ambient air at sea level. This failure may be classified as hypoxemic or hypercapnic, or even as acute or chronic. The main alterations may be hypoventilation, ventilation perfusion mismatch and diffusion defect. It may be secondary to a central origin, upper or lower airway compromise, parenchyma disease, or due to pleural or thorax wall affection. The hypoxemia evaluation may be done by hemoglobin saturation, alveolar capillary gradient of oxygen (D[A-a]O(2)) or by the paO(2) / FiO(2) index. COMMENTS: The knowledge of the physiopathologic mechanisms that cause respiratory failure in the child is important to define the more efficient therapeutic strategy for each cause.