Abstract: 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.