Abstract: A method of testing a material sample for defects, comprising the steps of applying a test signal to the material sample, receiving a return signal from the material sample and analysing the return signal to determine whether the material sample is defective. In one aspect of the present invention the steps of receiving a return signal and analysing it include the step of selecting a frequency range to reduce the effect of potentially interfering signals. In another aspect of the present invention a reference signal obtained from a non-defective sample is subtracted from the return signal to give a difference result and the difference result is indicative of whether the material sample is defective. Apparatus for carrying out the above-defined methods are also provided.