A method for detecting deposits in a pipe system of an apparatus that is flowed through by a fluid, wherein at least one microwave probe is introduced into the pipe system in such a way that the fluid flows against a window of the microwave probe that is transparent to microwave radiation, and wherein microwaves are coupled into the pipe system by way of the at least one microwave probe through the window that is transparent to the microwave radiation, the microwaves coupled into the pipe system propagating in the same way in the pipe system as in a hollow conductor, wherein a.) a reflection measurement is carried out, in which reflections of the microwaves radiated by the at least one microwave probe are received again by the respective microwave probe , cut-off frequencies being determined, for determining the cut-off frequencies the frequency of the microwaves coupled into the pipe system being varied in a preset region and the intensity of the reflected microwave radiation determined for each frequency and a comparison with a reference or a previous measurement being used to deduce a constriction in the surrounding area of the microwave probe , cut-off frequencies indicative of constrictions being detected and the free cross section at the constriction being determined from this cut-off frequency indicative of a constriction, and/or b.) at least two microwave probes are introduced into the pipe system at a distance from one another and a transmission measurement between two microwave probes is carried out, the arrangement of the two microwave probes defining a pipe system segment, cut-off frequencies of the pipe system segment being determined, for determining the cut-off frequencies the frequency of the microwaves coupled into the pipe system being varied in a preset region and the intensity of the reflected microwave radiation determined for each frequency and a comparison with a reference or a previous measurement being used to deduce a constriction in the pipe system segment, cut-off frequencies indicative of constrictions being detected and the free cross section at the constriction being determined from this cut-off frequency indicative of a constriction, wherein the detection of a constriction is used to deduce the presence of deposits. a.) a reflection measurement is carried out, in which reflections of the microwaves radiated by the at least one microwave probe are received again by the respective microwave probe , cut-off frequencies being determined, for determining the cut-off frequencies the frequency of the microwaves coupled into the pipe system being varied in a preset region and the intensity of the reflected microwave radiation determined for each frequency and a comparison with a reference or a previous measurement being used to deduce a constriction in the surrounding area of the microwave probe , cut-off frequencies indicative of constrictions being detected and the free cross section at the constriction being determined from this cut-off frequency indicative of a constriction, and/or b.) at least two microwave probes are introduced into the pipe system at a distance from one another and a transmission measurement between two microwave probes is carried out, the arrangement of the two microwave probes defining a pipe system segment, cut-off frequencies of the pipe system segment being determined, for determining the cut-off frequencies the frequency of the microwaves coupled into the pipe system being varied in a preset region and the intensity of the reflected microwave radiation determined for each frequency and a comparison with a reference or a previous measurement being used to deduce a constriction in the pipe system segment, cut-off frequencies indicative of constrictions being detected and the free cross section at the constriction being determined from this cut-off frequency indicative of a constriction, <EOS>