A nasal delivery device (11) for delivering substance to a nasal airway (1) of a subject, comprising: first and second nosepiece units (17, 19), each including a nosepiece (21, 23) for fitting to respective nostrils of a subject; at least one substance supply unit (13, 15) for supplying substance for delivery to the nasal airway (1) of the subject; and a valve unit (35, 37) for selectively fluidly connecting the at least one substance supply unit (13, 15) alternately to respective ones of the nosepiece units (17, 19).