Large area coating technology depends on reliable and accurate tools for in-line monitoring for production efficiency and quality. Although coating designs are becoming more and more complex, most deposition processes are still only controlled by parameters like electrical power, pressure, temperature and mass flow for which relatively cheap and reliable sensor technology is widely available. Nowadays, optical properties play a key role in the special performance characteristics of most coating designs. As a result, there is a growing need for more advanced control strategies in which optical properties are used as input parameters. The paper gives an overview of in-line optical characterization technology, practical solutions to measurement problems and examples of strategies for determining and controlling layer thickness, electric and dielectric properties and other coating material parameters. Among the examples that are discussed are strategies employed for controlling the deposition of a 4-layer AR coating on glass for building applications and that of a transparent conducting oxide layer on aluminum foil which is to be used as a superstrate in flexible thin film solar cell production. (c) 2005 Elsevier B.V. All rights reserved.