In this work, we consider White Light Reflectance Spectroscopy (WLRS) as an optical methodology for the accurate, fast and non-destructive measurement of film thickness in the 1 nm to the 1 mm range and for applications that include microelectronics, photonics, bioanalysis and packaging. Films to which WLRS is applicable can be either homogeneous or layered-composite ones, while thickness and composition might be fixed or arying with time; in the latter case, real-time monitoring of the kinetics of processes such as certain transitions, film dissolution and bioreactions is possible. We present the basic principles of WLRS and a selection of characteristic application examples of current interest, and we also briefly compare WLRS with alternative methods for film measurement.