Acoustic and Ultrasonic Methods for Particle Characterisation have many advantages. They are generally non-invasive, can be non-contact, are safe and often are economic. However, it can be difficult to interpret data, and the expertise and commercial equipment necessary may be in short supply and inappropriate to commercial requirements. Nevertheless the potential for these techniques is immense, particularly with regard to the newly emerging field of nanotechnology. A less well-recognised but just as important requirement is the ability to characterise systems on length scales between the molecular and the macroscopic. It is not so well known that acoustics can provide information over a huge range of length scales, from a few nanometres (ultrasound spectrometry) up to geological scales. Commonsense approaches to the understanding of acoustics obscure the potential of the modality. Scattering theory underpins all the theory of acoustic propagation, and adopting this initially theoretical approach indicates a world of new information. At one level particles and structures may be sized, at another their molar compressibility obtained, at another their shape determined. Acoustic methods are complementary to light scattering techniques offering advantages where light scattering does not work? optically opaque systems, mixtures with small refractive index differences, for example. So ultrasound spectrometry is uniquely well-suited to the characterisation of nanoparticle concentrates. In this article the theory of ultrasound propagation is outlined simply for a general audience, emphasising those aspects which provide the greatest potential for adoption of the modality in the particle characterisation community and briefly describing the relevant current commercial and laboratory equipment.