A measuring system for determining a torsion of a rotor blade, comprising a reference shaft to be arranged in the rotor blade in the longitudinal direction of the blade, at least one support for the reference shaft, for freely bearing the reference shaft in the rotor blade, so that the rotor blade can twist freely about the reference shaft, so that the reference shaft does not twist when there is twisting of the rotor blade, at least one rotary sensor, arranged on the reference shaft, for detecting a twisting of the rotor blade about the reference shaft in the region of the rotary sensor, the rotary sensor outputting a rotational angle describing the twisting of the rotor blade in relation to the reference shaft, or some other corresponding variable. A method for determining a torsion of a rotor blade, a corresponding rotor blade, a method for arranging a measuring system in a rotor blade and a corresponding wind power installation with a rotor blade and also or alternatively a measuring system.