Ubiquity and Abundance of H3+ in the Cosmos Takeshi Oka Depts of Chemistry and Astronomy & Astrophysics Enrico Fermi Institute University of Chicago In 1961, McDaniel speculated on the presence of H3+, in interstellar plasmas “to the virtual exclusion of H2+ and advocated its detection1. The 1980 discovery2 of the laboratory infrared spectrum of H3+ has enabled us to search for this important species in space. In 1989, the spectrum was detected in the auroral regions of Jupiter in emission, and subsequently also in Saturn, and Uranus3. In 1996, the spectrum was discovered in absorption4 toward young stellar objects that are deeply embedded in molecular clouds thus vindicating McDaniel’s prediction. In 1998, it was also observed in the diffuse interstellar medium contrary to theoretical predictions demonstrating the surprising ubiquity5 of H3+. The largest amount of H3+ has been detected toward the galactic center. Possible detections in a supernova, an active galactic nuclei, and a giant-gas protoplanet have also been claimed. I shall discuss H3+ which plays the most fundamental role in the physics and chemistry of those astronomical objects.