An aerosol delivery system comprises a container defining a chamber for a product to be delivered, an outlet from the chamber through which product may in operation be delivered, a valve for controlling passage of product through the outlet and a pump for pressurising product to be delivered, wherein the pump comprises an expandable material which, in operation, may be expanded to provide the pressure for pressurising product to be delivered, the expandable material being an osmotically effective agent and/or a swellable hydrogel and being disposed on one side of a semi-permeable membrane through which, in operation, fluid may be absorbed by the expandable material to expand it and thereby generate an osmotic pressure.