Abstract: A wind turbine is provided, having a wind turbine tower and at least one rotatable blade, and further comprising a system for measuring rotor blade vibration of said wind turbine. The system comprises at least one Doppler radar unit operatively configured to emit and receive radar signals, the radar unit being mounted on the wind turbine tower at a position above the lowest position of the at least one blade, the radar unit being positioned so as to measure reflections of an emitted radar signal from the turbine blade. A processing unit is configured to receive measurement data from the radar unit and to determine, by analysis of Doppler shift in received radar signals relative to transmitted signals due to movement of the blade towards or away from the turbine tower, the velocity of the blade in the direction towards or away from the turbine tower.