For the production of high performance multilayer systems optical monitoring is essential. Substantial progress was achieved by the introduction of direct monitoring on the rotating substrate holder. It is a complex task to develop a stable monitoring strategy for multilayers with a large number of layers and irregular thicknesses. The verification and improvement requires the feedback of more or less numerous real coating runs. This expensive and time consuming trial and error method can be reduced significantly by computer simulation of coating runs. A new software tool which simulates the coating process with monochromatic optical monitoring is introduced. Process instabilities are described by systematic and random errors of the deposition rate, refractive indices, etc. For the simulation of the monitoring curve real monochromatic bandwidth, signal noise, measurement frequency, etc. are taken into account. A UV-IR cut filter and a single notch filter design were simulated with virtual deposition runs. In both cases the simulation results were confirmed by real coating processes.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.