Molecular rotational motion is quantized, just as its vibrational and electronic states are quantized. Molecular rotational energy levels depend sensitively on the shape of the molecule and the masses of the constituent atoms. As a result, gas phase molecular rotational transitions, whose wavelengths are in the milIimeter and sub-millimeter region, provide signatures that allow chemical identification with exquisite selectivity. This tutorial will review the basics of molecular spectroscopy, then explore its application for chemical sensing. Specifically, a new double resonance spectroscopic technique will be described that exploits this remarkable specificity for remote chemical sensing. The technique overcomes intrinsic limitations of atmospheric attenuation and collisional broadening that have hindered other approaches. The hardware requirements and challenges for constructing such a spectrometer will be described.