The objective of this project is to improve homeland security by delivering a new technology which will allow to detect threats, explosives, pathogens and chemicals which are hidden by a person or inside an object such as letters or luggage. The new technology is based on THz radiation and advanced active and passive sensor concepts. In combination with existing sensors this will lead to an increased level of security at public places for example airports. This new class of sensors will support governments, agencies and public authorities in their effort to protect the public against terrorism. The technological objectives are: 1. To develop and demonstrate an active THz imaging system for remote detection of explosives and bioagents carried by a distant (ca. 25 m) person. The system is based on a cw THz laser for scanning the person and on a sensitive heterodyne scheme for the detection of the reflected laser radiation.2. To develop and demonstrate an active broad-band THz imaging system for close-by detection of explosives and bioagents hidden in objects such as letters. The system is based on a pulsed THz source with femtosecond lasers.3. To develop enabling technology which will allow to build compact, easy-to-use THz imaging systems. This includes sensitive detector-arrays and frequency stabilized THz quantum cascade lasers.