Terahertz wavelengths can pass through dry, non-polar, non-metallic materials that are opaque at visible wavelengths. Moreover they can be manipulated using millimeter wave and quasi-optical techniques to form an image. Sensing in this band potentially provides advantages in a number of areas of interest for security and defense, such as screening of personnel for hidden objects, and the detection of chemical and biological agents. This paper reviews recent research into THz applications by groups across Europe, the US, Australasia, and the UK. Several private companies are developing smaller and cheaper reliable devices allowing for commercialisation of these applications. While there are a number of challenges to be overcome there is little doubt that THz technologies will play a major role in the near future for advancement of security, public health and defense.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.