giant molecular clouds, their ages, and their confinement to spiral arms). 1977: Carbon monoxide is the best general-purpose tracer of molecular cloud mass. 1977: Galactic carbon monoxide emission peaks in a broad "molecular ring" at R~4 kpc. 1977/1994: Molecular clouds are mainly confined to a thin Gaussian layer ~100 pc wide, but a faint layer ~3 times as wide also exists. 1980/1983: Molecular clouds are excellent tracers of galactic spiral structure. 1980: Molecular clouds are relatively short-lived galactic objects. 1982/1983: The molecular cloud mass spectrum is steep, with most of the mass in the largest clouds. 1983: Intercomparision of carbon monoxide, HI, and diffuse gamma ray emissions provides perhaps the best large-scale calibration of carbon monoxide as a molecular mass tracer. The term X-factor was coined in this paper. 1985/1989/1991: Molecular clouds are dark nebulae both in the optical and the near infrared. 1986: Giant molecular complexes containing more than a million solar masses are not kinematic artifacts—as some had argued—but are well-defined objects that can be readily located throughout the galaxy. 1988: Roughly half of the interstellar gas within the solar circle is molecular. 2008: The enigmatic Expanding 3-kpc Arm has a Far 3 kpc symmetric counterpart on the far side of the Galactic Center. 2011: The Scutum–Centaurus spiral arm extends almost 360 degrees around the galaxy, from the end of the central bar to the warp near its outer edge. Personnel Prof. Patrick Thaddeus (the Robert Wheeler Willson Professor of Applied Astronomy, Emeritus, Harvard University; Senior Space Scientist, Smithsonian Astrophysical Observatory), who was leading the Millimeter-Wave group, died on April 28, 2017. Tom Dame (Radio Astronomer, Smithsonian Astrophysical Observatory; Lecturer on Astronomy, Harvard University) has coordinated telescope observations over the last decade. Sam Palmer (Electronics Engineer, Smithsonian Astrophysical Observatory; Lecturer on Astronomy, Harvard University) continues to maintain the telescope hardware. History Built by Thaddeus and colleagues in 1974, the telescope was operated from a Columbia University rooftop in Manhattan until it was moved to the CfA in 1986. Its twin instrument was constructed at Columbia and shipped to Cerro Tololo Inter-American Observatory, Chile in 1982. Observations of carbon monoxide had revealed that molecular gas in space was much more extensive than ever suspected. Initially, Thaddeus and his colleagues, Ken Tucker and Marc Kutner, had originally begun mapping the carbon monoxide using the sixteen-foot radio telescope at the McDonald Observatory in western Texas. The plan was to keep mapping