a delivery device for and a method of delivering a substance to the nasal airway 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 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 .