Abstract: The present invention relates to a method and apparatus for measuring material properties using guided-wave THz spectroscopy to measure the complex dielectric functions of materials. A high frequency wave is created and launched along a waveguide positioned in close proximity to a material under test. The wave is sampled initially, allowed to propagate, and sampled again. The samples are processed to derive information, such as resistivity, of the wafer. The method and apparatus of the present invention not only allows one to obtain information about the DC conductivity of metals, but also provides information about the high frequency behavior of materials—metals, and dielectrics as well—which is becoming particularly useful at the time when processor clocks are running at frequencies approaching GHz levels. The present invention provides for non-contact, non-destructive, non-contaminating testing.