The terahertz region of the electromagnetic spectrum is typically considered to occupy frequencies between 100 GHz and 4 THz (3 cm-1 to 130 cm-1; 3 mm to 75 µm); this bridges the gap between the microwave and millimeter region of the electromagnetic spectrum and infrared region. This region of the spectrum has been traditionally difcult to access, but recent developments in sources and detectors have seen a rapid commercialization of the technology. These advances have been accompanied by much interest in possible applications which has focused on the areas of pharmaceutical, security, and medical imaging.