Abstract: An opto-electronic focussing-error detection arrangement is described for the detection of a deviation between a radiation reflecting surface and the plane of focussing of an objective system. In the path of the beam which is reflected by the surface a beam-splitting element is disposed and behind said element two radiation-sensitive detectors are arranged which are each associated with one of the subbeams formed by the beam-splitting element. The detectors are grating-shaped detectors and are effectively divided into two detector sections by selection devices, the bounding line being adjustable. A focussing error signal is obtained which is highly independent of a positional error of the radiation-sensitive detection system relative to the beam axis.