The exploration and exploitation of the submillimetre and millimetre wave regions of the electromagnetic spectrum have progressed steadily over the last twenty years. Sophisticated Fourier spectroradiometric instrumentation, matching in performance that of the conventional instruments developed for the infra-red and visible regions over the last century, has become commercially available. Frequency can now be determined in this region directly against the caesium frequency standard to about 1 part in 1010, and the problem of reliable power measurement is being slowly resolved. The electrical properties of many materials have now been measured in this ultra-high frequency region, and the techniques are being successfully exploited in measurements of atmospheric composition and applied to the important area of plasma diagnostics.