The last two decades have seen the emergence of systems capable of delivering heated, humidified high-flow air and oxygen via nasal cannula to neonatal, paediatric and adult patients requiring respiratory support in emergency, critical care and general ward areas. These delivery systems (high-flow nasal cannula-HFNC) are regarded as an alternative to traditional oxygen delivery systems such as face masks and low-flow nasal cannula, and, increasingly, as a possible alternative to more invasive forms of oxygenation and ventilation such as non-invasive ventilation (NIV) and invasive mechanical ventilation. The HFNC delivery system consists of a gas blender and flow meter; an active humidifier; a heated inspiratory circuit; and the nasal cannula interface. HFNC provides a constant flow of humidified gas that can more accurately match the inspiratory flow of an acutely unwell patient whilst providing continuous positive airway pressure. Oxygen therapy is generally added into the flow and can be titrated to the patients’ oxygen requirements. Technical aspects of this therapy and considerations for its usage are outlined in this chapter. Further research is required to establish optimal HFNC commencement time frames and flow rates, in addition to early predictors of non-response to therapy so that appropriate treatment escalation is instituted.