The paper describes a comprehensive evaluation programme being carried out on optical fibre transmission systems, both in a field environment and also under controlled conditions in the laboratory. The work is providing detailed information on the performance of optical fibre systems from a number of commercial sources, in particular by monitoring the overall error performance and by measuring variations of parameters incorporated in the optical power budget. A data logging arrangement, which collects information from operational systems, and the laboratory test schedule are described. An overview of the results obtained to date is given, which concludes by validating the integrity of optical systems design.