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Until recently, the application of microwave systems has been hampered by the relatively large and expensive equipment, particularly in the millimetre and submillimetre-wave range. The development of more reliable and efficient solid-state sources up to the submillimetre-wave range has now stimulated new interest in commercial applications. Three different approaches can be distinguished, radar, radiometry and transmission measurement applications. The widest spread applications are found in radar systems for traffic control and industrial and scientific use. High resolution, high antenna gain and easy acquisition, even of non-metallic objects, in conjunction with contactfree or remote sensing, makes the millimetre-wave approach best suited for measurement purposes. Applications are the contactless measurement of velocity, distance and acceleration on rail vehicles, and radiometry in medicine and industry, e.g. for cancer diagnostics and atmospheric remote sensing. The behaviour of materials and gases can be examined, using the transmission measurement approach. Atmospheric pollution control and the non-destructive evaluation of solid materials are realizations of this technique. Employing millimetre- and submillimetre-wave frequencies for these applications gives inherent improvement of the measurement accuracy and due to the high transmission loss for this frequency range the risk of interference between different measurement systems is effectively reduced.