When light is very strongly multiply scattered by a medium, its propagation can be well described by a diffusion approximation. This allows important, measurable quantities to be calculated theoretically and interpreted. Thus, for example the total light transmitted through a sample can be used to determine the transport mean free path which characterizes the diffusive transport of the light. In addition, the distribution of path lengths followed by the diffusing light can be determined. This distribution can in turn be used to interpret the temporal fluctuations of the scattered intensity that arise due to the motion of the scattering medium. Therefore, traditional quasielastic, or dynamic, light scattering can be extended to the strongly multiple scattering limit. This technique is called Diffusing Wave Spectroscopy (DWS), and allows useful information about the dynamics of the medium to be determined. Furthermore, new physical processes can be studiedusing DWS. For example, DWS is sensitive to very small motions of colloidal particles: motion of 1 pm diameter particles can be resolved on lengths of∼ 5 Å using light with a wavelength of 0.5 μm. New physical phenomena are probed when motion on these length scales is observed. In particular, the time evolution of the hydrodynamic interaction between concentrated colloidal particles can be resolved. In addition, DWScan also probe spatially rare events since the light paths sample a large volume of the sample. This allows DWS to probe very slow dynamics, making it useful for the study of materials such as foams. This talk reviews the fundamentals of DWS and highlights some ofits unique applications.