Abstract: The invention relates to a method for measuring the thicknesses of thin layers by X-ray fluorescence, in which a specimen having the layer to be studied is positioned in view and subsequently X-radiation is directed onto the layer to be studied and emitted fluorescent radiation is detected by means of a radiation detector and the layer thickness is determined, in which on positioning the specimen there is a focussing by adjusting a focussing element along its optical axis and the position of the focussing element is determined with the layer in focus. An apparatus for layer thickness measurement with X-ray fluorescence according to the invention having a X-ray tube, a detector and an observing device with a focussing element provides for the latter to be movably mounted along its optical axis and provided with a position measuring device.