Ultrasonic standing waves can be used to generate radiation forces which act on particles within a fluid. These forces may be useful for manipulation, separation, or fractionation of these particles. This thesis describes the use and modelling of a number of devices which use such acoustic radiation forces. Initially, the design and testing of a large (multiple wavelength) flow-through ultrasonic separation device is described. This device demonstrated for the first time that flow-through ultrasonic filtration could be made to work successfully on this scale without having to rely on acoustically transparent membranes. Three models are used to examine the behaviour of the resonator. The electro-acoustic model is then used to analyse the performance of two smaller resonators and is shown to match experimental values well. The model is used to explain the behaviour of the resonators in the regions where individual layers of the device themselves have thickness resonances at similar frequencies. It also demonstrates the importance of the bonding between layers and shows that the standing wave at a peak of the energy density response differs, in terms of nodal position and boundary impedance, from simple, rigid-rigid boundary models. The remainder of the thesis concentrates on the design and use of micro-scale devices in which the scale of the layers is similar to or smaller than the wavelengths in use. A novel microfabricated filtration device is described which is primarily constructed using silicon and Pyrex. The modelling, design, fabrication and initial testing of the device are discussed. An expanded version of the electro-acoustic model allows prediction of the radiation forces on an example particle within a standing wave field. This is used to examine the force profile on a particle at resonances with pressure nodes at different positions. An analytical method for predicting modal conditions for combinations of frequencies and layer thickness characteristics is presented, which predicts that resonances can exist that will produce a pressure node at arbitrary positions in the fluid layer of such a system. The model also predicts conditions for multiple sub-wavelength resonances within the fluid layer of a single resonator, each resonance having different nodal planes for particle concentration. This forms the basis for the design of a unique microfabricated resonator with several modes that allow particles to be forced to either boundary of the fluid or to the fluid centre, depending on the operating frequency.