Abstract: A dry powder inhaler (10) consists of a chamber (12) into which powder is introduced from a metering system, and a mouthpiece (26) through which a patient inhales, connected by a duct (16), so that air flows through the chamber carrying the powder into the patient's lungs. Within the duct is an impactor (25) consisting of a plate (27) spaced in front of a similarly sized aperture, and between the chamber and the impactor plate is an orifice (18) narrower than the duct, so the air stream is diverted along S-shaped paths to avoid the plate. The presence of the impactor can improve the efficiency of delivery of a drug to the lungs, and decrease the unwanted dose to the mouth and throat, as the latter is principally due to agglomerates or larger particles which are intercepted or broken up by the impactor.