Sydlo, C. Precision Large Scale Synchronization System at the European XFEL JACoW Geneva, Switzerland Accurate timing synchronization on the femtosecond scale is an essential installation for time-resolved experiments at free-electron lasers such as FLASH and the upcoming European XFEL. The European XFEL imposes greater requirements due to its exceptional specifications, large number of stabilized optical fibers and a total length of 3.4km. The development towards a reliable system, their results and the current status of the optical synchronization system at the European XFEL are presented.