In this book chapter the authors have discussed the physical mechanisms explaining THz detection in FETs. Such detectors can support two modes of operations: (i) resonant mode and (ii) nonresonant mode. Even though resonant photo-detection is still to be fully demonstrated at room temperature, a lot of work has been done on nonresonant photo-detection using FETs. In this regard, nanometer size FETs operating as broadband THz detectors currently compete and are suitable alternatives to commercially available Schottky diodes. Recent studies show that FETs can reach responsivity levels, NEP, and speed of the same order of magnitude as Schottky diodes while having advantages arising from CMOS VLSI compatibility. In this regard, we have also presented an overview of recent experimental results on the detection of THz radiation by nanowire and GFETs.