Optical coatings have been in use for decades and a number of optical techniques have been used to characterize them but these all utilize only very short optical distances within the film. Guided optical beams can travel over 1 cm within even a thin film and this has significant advantages in determining the properties of the films. Very small losses in the film are measurable and the geometry is advantageous for evaluation of scattering phenomena. Refractive indices and film thickness can be calculated in a self-consistent manner to obtain better accuracy than is provided by any other technique for films and this has led to some surprising discoveries. Interference effects using guided beams allow extremely accurate measurement of film uniformity. The properties of extremely small quantities of liquids or gases on the film surface may also be examined. This paper discusses the basis for these techniques and their application to thin film characterization. The apparatus required to obtain accuracies comparable to or better than standard techniques is simple and inexpensive.