To deposit a thin film coating with the correct thickness and refractive index is very important for precision optics. There are several methods have been proposed to monitoring the thickness of each layer for a multilayer optical filter during the coating process. Among those methods, the optical monitoring is generally thought better than other methods to manufacture optical filters, because it has error compensations which cannot be provided in other monitor methods. For the manufacture of some delicate optical filters with the high defect rate, the optical monitor with higher monitoring precision and better error compensation ability is necessary. Several optical monitoring methods have been reviewed, and novel monitoring techniques have been introduced for precision optical coatings.