A method is described for the measurement of the thickness of thin films using the multiple beam interference techniques. Only simple equipment is required, but reduction of the order of interference to as low asonemakes possible the measurement of average thicknesses down to 10A. Applications have been made to evaporated films of six different metals and the method appears to be useful for all stable thin films in thicknesses up to several microns.Possible errors in the method and the precautions which may be taken to avoid them are discussed.