Many products of commercial significance exist as suspensions of particles in a liquid or will have consisted as such as suspension at a stage during manufacture; the particles may be solid or liquid. It is necessary to determine the physical state of such suspensions both in the development laboratory and for the purposes of quality and process control at plant level. Reliable estimates are required of the size distribution and concentration of the dispersed phase, as well as indications of flocculation, and network formation. Dynamic measures may be required in support of reaction processes such as crystallization. Techniques that can be used for the characterisation of suspensions are optical scattering or turbidity tests, sedimentation rate tests, ionizing radiation, electrical tests, electroacoustic measurements, and acoustic (ultrasonic) methods alone. Ultrasonic methods have the advantages that they can be used on mixtures that are too opaque for optical techniques, and that they can be incorporated into robust and low cost instrumentation. This paper gives a brief overview of the physics of the interactions of ultrasonic waves with particulate suspensions and a brief review of measurement methods and errors. Recent results that show agreement or otherwise between theory and experiment are given for silica sols. Examples are also given of the use of ultrasound to track flocculation in an aqueous emulsion, and to track a crystallization reaction.