Interstellar carbon monoxide (CO) was first detected in 1970 with the 36 foot diameter telescope of the National Radio Astronomy Observatory! on Kitt Peak in Southern Arizona. R. W. Wilson, K. B. Jefferts, and A. A. Penzias of Bell Labs reported, "We have found intense 2.6 mm line radiation 2 from nine Galactic sources which we attribute to carbon monoxide." Soon afterward, several other basic molecules were also observed in space. IAU Symposium 170, CO: Twenty Five Years of Millimeter Wave Spectroscopy, was organized to commemorate those discoveries. The Symposium reviewed the accomplishments of a quarter century of research on interstellar molec­ ular gas, surveyed the current state of millimeter-wave spectroscopy, and gave a glimpse of what the next 25 years might hold. Studies of interstellar CO have revolutionized our understanding of the phases and dynamics of the interstellar medium, the initial and final stages 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, some very distant. Spectroscopic studies of CO and other molecules are primary tools for investigating all these topics, which are among the most fundamental and active research areas in astrophysics. New developments in instrumentation, including sev­ eral powerful new telescopes, continue to keep millimeter and submillimeter wavelength radio astronomy at the forefront of research.

Present and Future CO Surveys.- CO in the Milky Way.- A Molecular Worm in Scutum.- Molecular Clouds Observed with the EGRET Gamma-ray Telescope.- CO and the Multiphase ISM.- The Ursa Major Molecular Clouds.- FCRAO CO Survey of the Outer Galaxy.- CO 2-1/1-0 Ratio.- Mapping Dust Extinction in Molecular Clouds.- On the Fractal Structure of Molecular Clouds.- The Origin and Structure of Molecular Clouds.- Molecular Gas Near the Galactic Center.- CO Mapping of the Inner Few Hundred Parsecs of the Galaxy.- Theoretical Chemistry.- Observational Chemistry.- CO at Other Wavelengths.- Millimeter-wave Absorption Studies of the Onset of Dark-cloud Chemistry.- Galactic Carbon Monoxide Isotope Ratios.- Using CO Isotopes to Probe the ISM.- Probing Giant Molecular Cloud Cores with Millimeter and Submillimeter Observations of C18O and Dust.- Velocity Coherence in Dense Cores.- Structure of Molecular Clouds and Turbulence.- 13CO (6-5) in the Orion Bar: A Critical Observational Test for PDR Models.- The C/CO Ratio Problem: Chemical Effects of Turbulence.- Turbulence and Collapse in Star-forming Molecular Clouds.- Star Formation in the Gem OB1 Molecular Cloud Complex.- Protostellar and Protoplanetary Disks.- 350 AU Scale Circumstellar Rotating Gaseous Disk around DM Tauri.- Kinematics of Disks around T Tauri Stars.- CO Outflows from Young Stars.- CO J=6?5 Observations of Protostellar Outflows.- Dense and Cold Gas in the Chameleon I Cloud: CO and IRAS Observations.- Surveys with Heterodyne Focal Plane Arrays at mm Wavelengths: QUARRY at FCRAO.- Single Dishes: The Potential of Present-Day Big Dishes and the Promise of the LMT.- Characteristics of the Millimeter Arrays.- Increasing the Yield of Our Telescopes.- Submillimeter Wavelength Fourier Transform Spectroscopy of AstronomicalSources.- The Submillimeter Telescope Observatory.- The Large Millimeter-wave Telescope.- Future CO Observations with the JCMT.- ODIN: A Swedish Submillimeter Wave Spectroscopy Satellite for Astronomy and Aeronomy.- European Plans for a Millimetre Array.- LMSA: Japanese Plans for a Large Millimeter and Submillimeter Array.- New Instruments, New Science: Future Opportunities.- Comparative Studies of Other Galaxies.- The ESO/SEST Key Programme: CO in the Magellanic Clouds.- CO in Nearby Normal Galaxies.- Molecular Gas and Star Formation in Galaxies.- Star Formation and Gas Contents in Disk Galaxies: Complex Relationships.- Molecular Gas in Ultraluminous Infrared Galaxies.- Molecular Clouds and CO Emission in Low-Metallicity Galaxies.- N(H2)/ICO in Galactic Bulges.- Molecular Gas in Galactic Nuclei.- Modeling of Molecular Emission from Centres of Galaxies.- Star Formation and Dynamics of Molecular Gas in Circumnuclear Regions of Barred Galaxies.- Structure of Circumnuclear Regions of AGNs: Star Formation and Dense Gas.- BIMA Observations of Molecular Gas in NGC 1068.- CO in High Redshift Galaxies.- CO (and Other) Lines from the Cloverleaf Quasar.- Centaurus A: The 13CO Map and Molecular Line Ratios.- CO Observations of High Redshift Galaxies with the Owens Valley Millimeter Array.- CO Observations of Ultraluminous Infrared Galaxies, QSOs, and Powerful Radio Galaxies with the 12m Telescope.- CO in the Solar System.- Mass Loss in AGB Stars.- Unraveling Mysteries in Late Stages of Stellar Evolution: The Enigma of the S-Type Stars.- New Detections of Metal-bearing Molecules in IRC+10216: From Chemistry to Nucleosynthesis.- CO in Planetary Nebulae and Proto-Planetary Nebulae.- Pre-Shock and Post-Shock Abundance Ratios of Atomic Carbon to CO in IC 443 G.- Twenty-five Years of CO Astronomy: Revealing the Cold Universe.- Poster Abstracts.- Author Index.- Object Index.- Participants.