A nasal delivery device for delivering substance to a nasal airway of a subject, comprising: a nosepiece for fitting to a nasal cavity of a subject; a mouthpiece into which the subject in use exhales; a delivery unit, which comprises an actuation part which is manually displaceable to actuate the delivery unit to deliver substance from the nosepiece; and a valve assembly which is fluidly connected to the nosepiece and the mouthpiece, wherein the valve assembly comprises a body element and a valve element which is movably disposed to the body element between closed and open configurations by manual displacement of the actuation part of the delivery unit to provide an air flow through the nosepiece simultaneously with delivery of substance; characterized in that: the body element includes a valve seat which defines a valve opening, and the valve element comprises a seal which acts to close the valve opening when the valve element is in the closed position and a support which supports a central region of the seal so as to allow a peripheral region of the seal to engage the valve seat and the central region to be deflected relative to the peripheral region, thereby allowing for sudden release of the seal, and whereby the valve assembly, on opening of the valve element, provides a burst of air flow, having a first, initial burst phase followed by a second, extended burst phase, wherein the peak flow rate in the first burst phase is at least 10% greater than that of the average flow rate of the second burst phase in a period corresponding to ten times the duration of the period in which substance is delivered from the delivery unit. a nosepiece for fitting to a nasal cavity of a subject; a mouthpiece into which the subject in use exhales; a delivery unit, which comprises an actuation part which is manually displaceable to actuate the delivery unit to deliver substance from the nosepiece; and a valve assembly which is fluidly connected to the nosepiece and the mouthpiece, wherein the valve assembly comprises a body element and a valve element which is movably disposed to the body element between closed and open configurations by manual displacement of the actuation part of the delivery unit to provide an air flow through the nosepiece simultaneously with delivery of substance; characterized in that: the body element includes a valve seat which defines a valve opening, and the valve element comprises a seal which acts to close the valve opening when the valve element is in the closed position and a support which supports a central region of the seal so as to allow a peripheral region of the seal to engage the valve seat and the central region to be deflected relative to the peripheral region, thereby allowing for sudden release of the seal, and whereby the valve assembly, on opening of the valve element, provides a burst of air flow, having a first, initial burst phase followed by a second, extended burst phase, wherein the peak flow rate in the first burst phase is at least 10% greater than that of the average flow rate of the second burst phase in a period corresponding to ten times the duration of the period in which substance is delivered from the delivery unit.