The terahertz (THz) region of the electromagnetic spectrum, broadly defined, comprises the range of frequencies from 100 GHz (10¹¹ Hz) to 10 THz (10¹³ Hz), corresponding to free-space wavelengths from 3 mm to 30 micrometers. This radiation, sometimes called “T-rays,” bridges the frequency gap between microwaves and long-wavelength infrared waves, and historically was of interest primarily to astronomers probing the cosmic microwave background, and fusion physicists diagnosing plasmas. This difficult transition region between radio frequencies and optics has become much more accessible in recent years with the development of new sources and detectors, spawning tantalizing potential applications for terahertz imaging in areas as diverse as hazardous-materials detection and security, nondestructive testing, medical imaging, and astronomy. As is the case in other wavelength regions, imaging is the natural way to retrieve spatial information contained in a terahertz scene.