Tunable far-infrared (FIR) radiation has been generated using CO2 laser difference generation in metal-insulator-metal diodes either from the difference between a fixed frequency CO2 laser and a tunable waveguide laser, or from the difference between two fixed frequency CO2 lasers plus microwave sidebands. Our tunable FIR source is being used to make highly accurate FIR frequency measurements of stable species to serve as frequency and wavelength calibration standards; to measure frequencies of transient species (including molecular ions) for astronomical searches; and to study line broadening and line shape parameters especially for atmospheric spectroscopy applications.