A nasal delivery device for delivering a substance to the nasal airway of a subject, comprising a delivery unit for delivering a gas flow and substance to one of the nostrils of the subject； wherein the delivery unit comprises a nosepiece which includes an outlet through which the gas flow and substance is in use delivered to the one nostril and a sealing member for providing a fluid tight seal between the outlet and the one nostril； wherein, during closure of the oropharyngeal velum of the subject, the delivery unit is configured to deliver the gas flow at a driving pressure to flow around the posterior margin of the nasal septum and out of the other nostril of the subject.
