During the past three years, molecules have been discovered in interstellar clouds with an abundance and chemical complexity totally unexpected by astrophysicists. Twenty-three molecules in a total of 34 isotopic combinations have now been identified through radio spectral line observations at more than 75 wavelengths from 2 millimeters to 36 centimeters. An important new chapter in astronomy has begun, with potential already demonstrated for providing new information and insights into such basic problems as the cosmic abundance of isotopes, formation of stars, structure of the Galaxy and the Galactic nucleus, and the thermodynamics of the interstellar medium. Equally important, many completely new phenomena have revealed themselves as it becomes clear that the most massive objects in the Galaxy are molecular clouds (see figure 1), and the physics of these regions is only now being explored. In addition an entirely new field, interstellar chemistry, is developing now that we know that chemical evolution of matter into substances as complex as organic molecules has taken place throughout the Galaxy.