The growing menace of international terrorism and the increasing number of terroristic attacks have drawn attention to new and more complex types of threats. Those must be countered by the development of new imaging technologies for wide area surveillance as well as for the detection of concealed dangerous objects, in order to protect the human population and sensitive important infrastructures. The observation of a variety of security critical premises, borders, and maritime coastal areas for instance attracts the increasing attention of companies and authorities. Hence there is a strong demand on wide-field-of-view imaging for intruder detection under all adverse ambient conditions. Furthermore the imaging of persons for security purposes is of increasing interest particularly for airline and other transportation services or public events with large crowds. Therefore personnel inspection with respect to weapons and explosives becomes an important mean. Passive microwave remote sensing allows a daytime independent non-destructive observation and examination of the objects of interest under nearly all weather conditions without artificial exposure of persons and observation areas, hence fully avoiding health risks. The penetration capability of microwaves provides the detection of objects through atmospheric obstacles like bad weather, fog or dust, vapour and smoke, as well as through thin non-metallic materials and clothing. For the latter the detection of hidden objects like weapons, explosives, and contraband is possible by monitoring dielectric anomalies. Based on the physical principles of microwave radiometry, images have a quasi-optical appearance simplifying the image interpretation for the operator. In addition, the sensor operation is inherently passive and covert. Over years the DLR Microwaves and Radar Institute developed several problem-orientated radiometer imaging systems covering nearly the whole frequency spectrum between 1 GHz and 100 GHz for a multitude of applications. Mechanical linescanner systems, having a different field of view for far-field or near-field imaging, are used as experimental systems in order to investigate basic key parameters for the specific applications like suitable frequency band, required spatial resolution, and sensitivity. The different types of imaging systems are introduced and some typical measurement results are shown. Besides laboratory results, some measurements are discussed, which were recorded under nearly operational conditions like the surveillance of facilities and the personnel screening at a security checkpoint.