The field of interstellar molecules is reviewed with special consideration of recent observational and technical advances in the shorter sub-millimeter wave region of the electromagnetic spectrum. At present more than 110 interstellar molecules have been identified in interstellar clouds and circumstellar envelopes. The more complex molecules are found in the dense cores which are often the sites of active star formation. These locations represent prime targets for the search of larger molecules such as glycine. However, in the list of detected interstellar molecules many simple hydrides are still missing, including, e.g., SH, PH, PH 2, etc., which constitute the building blocks of larger molecules. With the technological opening of the terahertz region ( v ≅ 1 THz corresponds to λ ≅ 0.3 mm) to both laboratory and interstellar spectroscopy, great scientific advances are to be expected. Amongst these will be the direct detection of the lowest rotational transitions of the light hydrides, the low energy bending vibrations of larger (linear) molecules, and possibly the ring-puckering motion of larger ring molecules like the polycyclic (multiring) aromatic hydrocarbons.