Terahertz electromagnetic radiation is one of the last remaining unexplored regions of the electromagnetic spectrum; occupying a large portion of the spectrum_between the infrared and microwave bands. Until relatively recently, the THz region (0.1-3 THz, 0.1-3 mm) has remained in relative obscurity due to a lack of efficient laboratory emitters, detectors and optical components compared to neighboring microwave and optical bands. This is despite the fact that many important processes in nature occur at THz frequencies: for example, the vibrational breathing modes of many large molecules occur at these low frequencies, giving rise to a unique 'fingerprint' for many biomolecules in the THz region. Indeed, it has been proposed that one of the most important future applications of THz radiation will be in biomedicine. In the future the THz region will be as useful as the microwave and infrared frequency bands are today.