Terahertz (THz) electronics using mainstream CMOS technologies can be a small, low-cost alternative to discretecomponent THz systems. Due to high yield and integration level, large-scale THz imaging systems can be affordably realized in a small form factor. In this paper, state-of-the-art CMOS circuits for THz imaging are reviewed. Incoherent detectors in CMOS process offer comparable noise equivalent power (NEP) to III-V counterparts at a fraction of the cost. An 820-GHz 8×8 array with minimum NEP of 12.6pW/√Hz is demonstrated using diode-connected MOSFET’s in 130- nm CMOS. Schottky-barrier diodes (SBD’s) fabricated using a 130-nm CMOS process demonstrate higher cutoff frequency than MOSFET’s. Using the SBD, detection at 9.7THz is demonstrated. The same SBD’s are also used to implement a 218-GHz 6×6 detector array for a THz camera module. Mixer-based coherent detectors show orders-ofmagnitude better sensitivity than that of incoherent detectors. Mixers require a local oscillator (LO) signal. The design challenge of including an LO can be relaxed by using a sub-harmonic mixing technique. A 410-GHz 4th order subharmonic mixer requires −1.6-dBm LO power at 102.5GHz and shows 44-dB better sensitivity than incoherent detectors operating near 400GHz. LO’s can be directly integrated with the mixing device to form a compact transceiver. A 260-GHz transceiver that integrates a VCO, antenna and mixer, occupies only 480×580μm<sup>2</sup> and shows a 13.5-dB better sensitivity at 260 GHz than the incoherent detector with the lowest NEP. Since the area is less than λ/2×λ/2, it should be possible to build large-scale focal plane arrays with coherent detectors and transmitters.