Colloidal suspensions and biological macromolecules often appear turbid or coloured-opaque, since the incident light is scattered many times by particles or inhomogeneous structures resulting in diffusion of light waves. The random distribution of scatterers gives rise to complex interference patterns known in optics as speckle patterns. Speckle patterns of strongly scattering media show novel correlations over a wide angular range and the analysis of their temporal fluctuations provides information on the motions of the scatterers. This novel technique called "Diffusing Wave Spectroscopy" has found diverse applications, for example in particle sizing, in the on-line quality control of dispersions, in the determination of flow velocity profiles or in medical imaging.