The influence of a transverse surface crack on the dynamic behaviour of a cantilever beam is studied. Analytical and experimental investigations provide a link between the change in natural frequencies of vibration and in mechanical impedance to the location and size of the crack for flexural vibrations. It is shown that the mechanical impedance of the beam changes substantially due to the presence of the crack and can be used as an additional defect information carrier. The results have been used to propose an improved method of non-destructive testing for simple beam structures. The method is based on the combined examination of the change in natural frequencies and in mechanical impedance and allows an estimation of both location and size of the crack.