Interstellar carbon monoxide (CO) was first detected in 1970 by R. W. Wilson, K. B. Jefferts, and A. A. Penzias through observations of its lowest rotational transition at 2.6 mm wavelength. This discovery opened the door to a profound new understanding of several diverse yet related fields, including the phases of the interstellar medium, the initial and final phases of stellar evolution, the chemistry of dense and diffuse interstellar matter and of the solar system, the structure of the Milky Way galaxy, and the content and structure of other galaxies. These are among the most fundamental...