Abstract: A device and an evaluation procedure for the depth-selective, non-invasive detection of the blood flow and/or intra- and/or extracorporeally flowing liquids in biological tissue are described whereby photons of a coherent, monochromatic source of light are entered into the tissue through a first area, photons reemerging from the tissue at different distances from this first area are detected with respect to their frequency and number, or intensity, and, from this information, i.e. frequency and/or number, or intensity, and/or reemerging location, conclusions about the relative modification of the flow amount and/or speed and/or location of the blood flow and/or the intra- and/or extracorporeally flowing liquids in the tissue are drawn with the help of an evaluation program or algorithm.