Precise thickness monitoring concepts are of key importance for the successful manufacturing of optical coatings, not only, but especially in case of highly complex thin film designs. This course provides an overview of the different thickness monitoring approaches available today, comprising the basic principles as well as examples of technical implementations. Besides some aspects of conventional non-optical and optical monitoring solutions in particular modern optical monitors covering a broad spectral range are in focus. The course gives background on different monitoring strategies including examples of thickness determination algorithms applied in recent developments. The advantages of direct monitoring concepts will be discussed and illustrated by practical examples. Regarding technical aspects, the essential parts of thickness monitoring systems will be presented comparing different alternative solutions. This comprises typical light sources, optical components, and spectrometers for different wavelength ranges from the UV to the NIR. In addition, adapted monitoring configurations and error handling options as well as hybrid monitoring strategies based on the application of more than one thickness determination approach will be discussed. Also, some aspects of the process control interfaces of the monitoring systems and the deposition plants will be addressed. Furthermore, detailed information on the integration of thickness monitors in flexible manufacturing concepts will be given. In these adaptive manufacturing environments, tailored computational manufacturing tools are combined with monitor specific on-line re-calculation and on-line design re-optimization modules. Examples will be presented to demonstrate resulting advantages as highest precision and flexibility, increased economic efficiency, and shortest product development times.