Within the scientific conception of the modern world, thin film optical coatings can be interpreted as one-dimensional photonic crystals. In general, they are composed of a sequence of single layers which consist of different transparent dielectrics with a thickness in the nanometer scale according to the operation wavelength range. The major function of these photonic structures is to adapt the properties of an optical surface to the needs of specific applications. By application of optical thin film coatings with optimized designs, the spectral characteristics of a surface can be modified to practically any required transfer function for a certain wavelength range. For example, the Fresnel reflection of a lens or a laser window can be suppressed for a broad wavelength range by depositing an antireflective coating containing only a few single layers. On the basis of a layer stack with alternating high- and low-refracting materials, high reflectance values up to 99.999\% can be achieved for a certain laser wavelength. In addition to these basic functions, optical coatings can realize a broad variety of spectral filter characteristics according to even extremely sophisticated demands in modern precision optics and laser technology. Moreover, recent developments in optical thin film technology provide the means to combine selected optical properties with other features concerning, for instance, the thermal, mechanical or chemical stability of a surface. The latest progress in ophthalmic coatings even includes the integration of self-cleaning, photoactive or anti-fogging functions in antireflective coatings on glass. As a consequence of this enormous flexibility in adjusting the properties of functional surfaces, optical coatings can be found in nearly every product and development of modern optic today. In order to keep pace with the rapid development of optical technology, innovations in the design, deposition processes and handling of optical coatings are some of the crucial factors. Also, high demands in respect to precision and reproducibility are imposed on the control of layer thickness during the production of the coating systems. For certain applications in fs lasers or optical measurement systems the individual layer thickness has to be controlled within the sub-nanometer scale, which can be only achieved on the basis of advanced in situ monitoring techniques of the growing layers. These skills have to be complemented by extended knowledge of characterization, because optimization and marketing of optical coatings can only be performed on the basis of reliable and standardized characterization techniques. The present chapter addresses these major aspects of optical coatings and concentrates on the essential topics of optical coatings in their theoretical modeling, production processes, and quality control.