A non-destructive testing method of analyzing a sample comprising a composite material is disclosed. The method comprises: emitting an electromagnetic signal onto the sample, the electromagnetic signal having a range of frequencies; detecting a response signal transmitted and/or reflected by the sample in response to the electromagnetic signal; processing the response signal to determine variation with frequency of a dielectric permittivity of the sample over the range of frequencies; and determining an indication of a structural characteristic of the sample from a measure of the variation with frequency of the dielectric permittivity of the sample.