A delivery device (20, 22) for and a method of delivering a substance to the nasal airway (1) of a subject, in particular the posterior region of the nasal airway, the delivery device comprising: a closure unit for causing the closure of the oropharyngeal velum of the subject; and a delivery unit for delivering a gas flow entraining a substance to one of the nostrils of the subject at such a driving pressure as to flow around the posterior margin of the nasal septum and out of the other nostril of the subject, wherein the delivery unit comprises a nosepiece (30, 40, 58, 82, 102, 132) which includes an outlet through which the gas flow is in use delivered to the one nostril and a sealing member for sealing the one nostril to the outlet such as in use to prevent the escape of the gas flow through the one nostril.