Complex respiratory events, which may have a detrimental effect on both quality of sleep and control of nocturnal hypoventilation, occur during sleep in patients treated with non-invasive ventilation (NIV). Among these events are patient-ventilator asynchrony, increases in upper airway resistance (with or without increased respiratory drive) and leaks. Detection of these events is important in order to select the most appropriate ventilator settings and interface. Simple tools can provide important information when monitoring NIV. Pulse oximetry is important to ensure that adequate oxygen saturation is provided and to detect either prolonged or short and recurrent desaturations. However, the specificity of pulse oximetry tracings during NIV is low. Transcutaneous capnography helps discriminate between hypoxaemia related to ventilation/perfusion mismatch and hypoventilation, documents correction of nocturnal hypoventilation and may detect ventilator-induced hyperventilation, a possible cause for central apnoea/hypopnoea and glottic closure. Data provided by ventilator software help the clinician by estimating ventilation, tidal volume, leaks and the rate of inspiratory or expiratory triggering by the patient, although further validation of these signals by independent studies is indicated. Finally, autonomic markers of sympathetic tone using signals such as pulse wave amplitude of the pulse oximetry signal can provide reliable information of sleep fragmentation.