The invention relates to a method and a device for monitoring the state of rotor blades on wind power installations. According to the invention, structure-borne noise is measured by means of at least one displacement sensor arranged on the rotor blade, the output signals of said sensors determine a frequency spectrum in an evaluation unit by means of suitable methods, the frequency spectrum is compared to reference spectra corresponding to defined states of damage and other particular states and stored in the evaluation unit, and the state of the rotor blade is determined therefrom. The aim of the invention is to provide a method and a device which enable existing local inner and outer damage, and other particular damage-causing states of the rotor blades, to be rapidly identified and evaluated, in order to take into account the effect thereof on the operation of the installation in an automatic manner. To this end, structure-borne noise is measured from a sequence of individual measurements carried out directly one after the other, the beginning of each of said individual measurements being triggered according to a uniform rotational angle of the rotor blade in relation to the rotational axis of the rotor, and all of the determined frequency spectra of each individual measurements are accumulated before being compared with the reference spectra.