A new fast scan submillimeter spectroscopic technique (FASSST) has been developed which uses a voltage tunable backward wave oscillator (BWO) as a primary source of radiation, but which uses fast scan(∼105Doppler limited resolution elements/s) and optical calibration methods rather than the more traditional phase or frequency lock techniques. Among its attributes are (1) absolute frequency calibration to∼1/10of a Doppler limited gaseous absorption linewidth(<0.1 MHz, 0.000 003 cm−1), (2) high sensitivity, and (3) the ability to measure many thousands of lines/s. Key elements which make this system possible include the excellent short term spectral purity of the broadly(∼100 GHz)tunable BWO; a very low noise, rapidly scannable high voltage power supply; fast data acquisition; and software capable of automated calibration and spectral line measurement. In addition to the unique spectroscopic power of the FASSST system, its implementation is simple enough that it has the prospect of impacting a wide range of scientific problems. ©1997 American Institute of Physics.