A portable breathing assistance device, comprising: a housing which defines a mouthpiece; and a connector for receiving a portable source of pressurized gas at a pressurization level, for providing gas to the user at a pressure above atmospheric pressure, wherein the device further comprises: the portable source of pressurized gas, comprising a portable gas canister; and a pressure control system coupled to an output from the gas canister for providing pressure control at least during exhalation to provide a positive end-expiratory pressure, characterized in that the pressure control system comprises a pressure reducer so that the pressure is controllable to a pressure below the pressurization level, in that the pressure control system is further controllable to regulate different pressures during inhalation and exhalation, and in that the pressure control system comprises a first valve between the source of pressurized gas and the mouthpiece, a second valve (116; 120) between the mouthpiece and a vent to ambient air and a pressure feedback path (108,110,112,114; 134) for controlling the second valve to regulate the pressure at the mouthpiece in dependence on whether the first valve is open or closed. a housing which defines a mouthpiece; and a connector for receiving a portable source of pressurized gas at a pressurization level, for providing gas to the user at a pressure above atmospheric pressure, wherein the device further comprises: the portable source of pressurized gas, comprising a portable gas canister; and a pressure control system coupled to an output from the gas canister for providing pressure control at least during exhalation to provide a positive end-expiratory pressure, characterized in that the pressure control system comprises a pressure reducer so that the pressure is controllable to a pressure below the pressurization level, in that the pressure control system is further controllable to regulate different pressures during inhalation and exhalation, and in that the pressure control system comprises a first valve between the source of pressurized gas and the mouthpiece, a second valve (116; 120) between the mouthpiece and a vent to ambient air and a pressure feedback path (108,110,112,114; 134) for controlling the second valve to regulate the pressure at the mouthpiece in dependence on whether the first valve is open or closed. the portable source of pressurized gas, comprising a portable gas canister; and a pressure control system coupled to an output from the gas canister for providing pressure control at least during exhalation to provide a positive end-expiratory pressure, characterized in that the pressure control system comprises a pressure reducer so that the pressure is controllable to a pressure below the pressurization level, in that the pressure control system is further controllable to regulate different pressures during inhalation and exhalation, and in that the pressure control system comprises a first valve between the source of pressurized gas and the mouthpiece, a second valve (116; 120) between the mouthpiece and a vent to ambient air and a pressure feedback path (108,110,112,114; 134) for controlling the second valve to regulate the pressure at the mouthpiece in dependence on whether the first valve is open or closed.