Ultrasound spectrometry is one of very few methods that can size particles over the range 10 nm to 1 mm for optically opaque,undiluted materials.It has in-line,non-invasive capabilities which make it a serious contender for use in industrial process monitoring,with the potential for 100%inspection.In aqueous systems,it is more sensitive to nanoparticles than to particles in the micrometre range upwards because the scattering power varies as the inverse square of particle diameter,making it suitable for the detection of nanoparticles in concentrated,mixed systems.Future development of ultrasound spectrometers suitable for widespread laboratory and industrial use depends on meeting the challenges of complex data interpretation and the need for considerable know how.In this review we provide a brief account of ultrasound propagation and scattering theory which underlies the ultrasound spectrometer,describe several types of spectrometer and demonstrate its successful use in the characterization of colloidal silica,whole milk and protein solutions. Ultrasound spectrometry is one of very few methods that can size particles over the range 10 nm to 1 mm for optically opaque, undiluted materials. It has in-line, non-invasive capabilities which make it a serious contender for use in industrial process monitoring, with the potential for 100% inspection.In aqueous systems, it is more sensitive to nanoparticles than to particles in the micrometre range upwards because the scattering power varies as the inverse square of particle diameter, making it suitable for the detection of nanoparticles in concentrated, mixed systems. Future development of ultrasound spectrometers suitable for widespread laboratory and industrial use depends on meeting the challenges of complex data interpretation and the need for for know know. In this review we provide a brief account of ultrasound propagation and scattering theory which underlies the ultrasound spectrometer, describe several types of spectrometer and demonstrated its successful use in the characteriz ation of colloidal silica, whole milk and protein solutions.