It is necessary to take into account scattering phenomena in optical systems since one is interested in a very detailed energy balance. We recall the principles of the vector theories that have been developed to predict the spatial and spectral distribution of light scattered from an optical surface or a multilayer coating. With the help of a scattering apparatus, we emphasize the key scattering parameters (roughness, autocorrelation length, isotropy degree, crosscorrelation laws) that are necessary to characterize surface defects and materials microstructure in thin film form. In spite of numerous parameters involved in the calculation, particular techniques enable us to extract all these parameters and lead to good agreement between theory and experiment. When the coating is made of a high number of layers, investigation of experimental results is quite more difficult. However, with the help of a correctly chosen model, we can strongly reduce the number of scattering parameters, and this allows an easy comparison between theory and experiment.