of non-destructive (NDT) testing in civil engineering. More on microwave imaging in NDT is described in the following. Microwave testing Microwave testing uses the scientific basics of microwave imaging for the inspection of technical parts with harmless microwaves. Microwave testing is one of the methods of non-destructive testing (NDT). It is restricted to tests of dielectric, i. e. non-conducting material. This includes glass-fibre reinforced plastic (GRP, GFRP). Microwave testing can be used to inspect components also in a built-in state, e. g. built-in non-visible gaskets in plastic valves. Principle The microwave frequencies extend from 300 MHz to 300 GHz corresponding to wavelengths between 1 m and 1 mm. The section from 30 GHz to 300 GHz with wavelengths between 10 mm and 1 mm is also called millimeter waves. Microwaves are in the order of the size of the components to be tested. In different dielectric media they propagate differently fast and at surfaces between them they are reflected. Another part propagates beyond the surface. The larger the difference in the wave impedance, the larger is the reflected part. In order to find material defects, a test probe, attached or in a small distance, is moved over the surface of the device under test. This can be done manually or automatically. The test probe transmits and receives microwaves. Changes of the dielectric properties at surfaces (e. g. shrinkage cavities, pores, foreign material inclusion, or cracks) within the interior of the device under test reflect the incident microwave and send a part of it back to the test probe, which acts as a transmitter and as a receiver. The electronic data evaluation leads to a display of the results, e. g. as a B-scan (cross sectional view) or as a C-scan (top view). These display methods are adopted from ultrasonic testing. Procedures Besides the reflection method also the through transmission method is possible, in which separate transmit and receive antennas are used. The backside of the device under test (DUT) must be accessible and the method gives no information about the depth of a defect within the DUT. Microwave tests are possible with constant frequency (CW) or with continuously tuned frequency (FMCW). FMCW is advantageous to determine the depth of defects within the DUT. A test probe attached to the DUT's surface gives information about the material distribution below the point of contact. When moving over the DUT surface point by