antenna in bistatic configurations. To increase the cross-range resolution of the imaging system, several antennas should be distributed over an area (which is called the sampling area) with a spacing less than the operating wavelength. However, the mutual coupling between the antennas, which are placed close to each other, may degrade the accuracy of the collected signals. Moreover, the transmitter and receiver system will become very complex. To address these problems, one single scanning antenna is used instead of several antennas. In this configuration, the antenna scans over the entire sampling area, and the collected data is mapped together with their antenna position coordinates. In fact, a synthetic (virtual) aperture is produced by moving the antenna (similar to the synthetic aperture radar principle). Later, the collected data, which is sometimes referred to as raw data, is fed into the software for processing. Depending on the applied processing algorithm, microwave imaging techniques can be categorized as quantitative and qualitative. Applications Microwave imaging has been used in a variety of applications such as: nondestructive testing and evaluation (NDT&E, see below), medical imaging, concealed weapon detection at security check points, structural health monitoring, and through-the-wall imaging. Microwave imaging for medical applications is also becoming of more interest. The dielectric properties of malignant tissue change significantly in comparison with the properties of normal tissue (e.g., breast tissue). This difference translates into a contrast which can be detected by microwave imaging methods. As one example, there are several research groups all around the world working on developing efficient microwave imaging techniques for early detection of breast cancer. Ageing of infrastructure is becoming a serious problem worldwide. For example, in reinforced concrete structures, corrosion of their steel reinforcements is the main cause of their deterioration. In U.S. alone, repair and maintenance cost due to such corrosion is about $276 billion per year, [3]. Recently, microwave imaging has shown great potential to be used for structural health monitoring. Lower frequency microwaves (e.g., <10 GHz) can easily penetrate through concrete and reach objects of interest such as reinforcement bars (rebars). If there is any rust on the rebar, since rust reflects less EM waves in comparison with sound metal, the microwave imaging method can distinguish between rebars with and without rust (or corrosion). Microwave imaging also can be used to detect any embedded anomaly inside concrete (e.g., crack or air void). These applications of microwave imaging are part