Optical monitoring (OM) during film production is considered for thermal vacuum and spin coating processes. During spin coating, OM has allowed identification of several phases during film formation, and characterization of the temporal dynamics may improve control and reproducibility in film production. For films grown under high vacuum, optical monitoring allows continuous inspection of the interference signal, with typical determination of optical thicknesses in multiples of quarterwaves. Long trial-and-error procedures are avoided, even under presence of density fluctuations during deposition. Instrumentation and metrologic aspects are presented. Examples are given of liquid film deposition for silicate sol-gel films and silicone oils, as well as of thermal evaporation of ZnSe and ZnS films. In these, we also performed an intercomparative analysis with the post- process, spectrophotometric envelope method. Optical interferometry monitoring is non-destructive, can be integrated to film manufacturing processes, allows precise metrological procedures, and can be applied to the real-time and in situ characterization of a wide variety of non- opaque, liquid or solid, films.