Being able to predict the reflected pressure from a surface, whether that is a reflector, a diffuser, or a noise barrier, enables efficient evaluation, design, and characterization of the performance. Currently, for reflectors and diffusers, this is usually done by considering the surface in the free field, in isolation from other objects and boundaries. The prediction techniques outlined in this chapter could, in theory, be used to predict the sound for whole rooms or outdoor environments such as street canyons (if the effects of weather are ignored). At the moment, however, long computation times and storage limitations mean that algorithms dealing with large spaces are forced to use cruder representations of reflection and diffraction. Consequently, when predicting the responses in rooms and semi-enclosed spaces, such as pavilions, it is more common to use geometric models. The issue of modelling scattering in geometric room acoustic models (GRAMs) is discussed in Chapter 13.