Heterodyne spectroscopy can achieve resolving powers of more than 106 in the far-infrared spectral region. However, in the range 30-300μm (10-1THz), there are problems with both detectors and local oscillator (LO) sources. At longer wavelengths GaAs Schottky barrier diodes have been employed with great success, using optically-pumped far-infrared lasers (OPFIRLS), carcinotrons or harmonic generation from millimetre wave sources as LOs. Superconductor-Insulator-Superconductor (SIS) mixers, which require much less LO power than Schottky diodes, are excellent devices at wavelengths longer than 600μm. A great advantage of all rectifying detectors is the very large intermediate frequency (IF) bandwidth available. Typically this might be ±20GHz, allowing spectroscopy over a useful wavelength range (eg ∆λ = 12μm at 300μm) even with fixed frequency LOs.