The advantages of Fourier Spectroscopy in the far infrared can now be realised for routine investigation. Michelson and Lamellar grating interferometers designed to complement each other in the far infrared, together with a special purpose computer, give a simple, easily operated system. Some of the design criteria for a commercial system are discussed and results are shown at resolving powers from 10 cm -1 to 0.25 cm -1.