Abstract: This invention relates to apparatus and methods for sensing terahertz radiation, in particular over an area, and to terahertz radiation imaging systems. A terahertz radiation sensor, the sensor comprising an optical beam input to receive an optical probe beam, a detector to modulate said probe beam responsive to terahertz radiation, and a photosensitive detector to provide an output responsive to said probe beam modulation. The sensor being configured to provide a first optical path between said optical beam input and said electro-optic detector and to provide a second optical path between said electro-optic detector and said photosensitive detector, and wherein said sensor further comprises a polarizer, said polarizer being located in both said first and said second optical paths. We further describe imaging systems for use with such a probe.