A nasal delivery device and a method of delivering substance to a nasal airway of a subject can be used for mass treatment, especially mass vaccination. The delivery device includes an interface unit, as a replaceable unit, having at least one nosepiece unit for fitting to a respective nostril of a subject, a nozzle from which substance is in use delivered, and at least one delivery unit having a substance supply unit for delivering substance to the nozzle of the at least one nosepiece unit. The delivery device also includes an actuation unit for actuating the at least one delivery unit of the interface unit.