The millimeter (MM) wave and sub-MM wave (30-600 GHz) frequency band contains fundamental rotational and vibrational resonances of many molecular gases composed of carbon, nitrogen, oxygen, and sulphur. The high specificity of rotational spectra to organic molecules affords MM wave spectroscopy having potential use in remotely sensing atmospheric pollutants and the detection of airborne chemicals is important for arms control treaty verification, intelligence collection, and environmental monitoring. This paper considers the sensitivity requirements of radiofrequency receiver systems for measuring MM wave absorption/emission signatures. The significance of receiver sensitivity and material optical depth to sensing is highlighted. A background to the technology needed for sensing at MM and sub-MM wavelengths then provides the basis for a review of MM wave spectroscopy and its role on profiling the concentrations of trace polar molecules and ionized radicals in the high altitude atmosphere. The application of the MM wave spectroscopic technique in ambient conditions is then reviewed and the issues associated with developing the technique for standoff remote sensing is discussed.