Summary of the introductory presentation for a panel discussion on "Characterization of optical thin film" The determination of the refractive index and the extinction coefficient of optical thin films can be nowaday higly accurate wheter carried out by skilled operators. Reliable results can be obtained even for inhomogeneous layers or for films with non-uniform thickness and measurements during the film deposition can also be performed. The accuracy of results, however, depends on the instrument performances in addition to the operator skill. In fact high accuracies require complex and expensive measurements apparatus, often "ad hoc" made, and data processing must be performed by people able to understand the physical meaning of the obtained results. Often a very high accuracy on the determination of optical constants is not required provided easy and fast measurements methods can be employed. In these cases a careful choice of the method and the relative measurements apparatus is necessary and some practical clever-nesses must be used for avoiding those mistakes possible even though advanced instrumentation is employed. In the present discussion the techniques most widely used for n and k determination will be surveyed and some practical suggestions will be made aimed at ensuring that measurements and data processing are performed correctly.