A delivery device for and method of delivering substance, in particular a vaccine, to a mucosal surface within the oral cavity of a subject, the device comprising: a mouthpiece unit to be gripped in the mouth of a subject, wherein the mouthpiece unit is configured such that, on exhalation or attempted exhalation by the subject, a pressure is developed in the oral cavity which is such as to close the oropharyngeal velum of the subject； and an oral outlet unit including at least one outlet from which substance is in use delivered to a mucosal surface within the oral cavity of the subject.
