Abstract: The invention relates to a method for length and/or velocity measurement, in particular for positioning tasks, in which an optical sensor carries out a length and/or velocity measurement on a measurement object in a contactless manner, wherein the optical sensor carries out the length and/or velocity measurement via an image processing method, via a spatial frequency filter method and/or by way of a laser Doppler method and reference markings are recognized by the optical sensor, and to a corresponding apparatus. The object of providing a simple method and a simple apparatus for length and/or velocity measurement which offers a higher degree of measurement reliability with the result that it can also be used in safety-relevant areas of application is achieved by evaluation means carrying out reference marking recognition operations and by a plausibility test being carried out, in which a signal is generated as a function of the result of the plausibility test.