This summary of H3+ in astronomy is based on an invited symposium that I gave at the IAU last August. Originally a laboratory curiosity, the molecular ion H3+ was recognized as a constituent of interstellar molecular gas in 1961, was proposed in the early 1970s to be the principal instigator of gas phase interstellar chemistry, and was observationally demonstrated as such two decades later. H3+ is the dominant infrared line emitter in the aurorae of three of the four gas giant planets in the solar system and is expected to produce bright line emission in the upper atmospheres of giant exoplanets located close to their stars. Infrared spectroscopy of H3+ in the interstellar medium is providing constraints on the spectrum of low energy cosmic rays in the Galaxy, has led to the discovery of an important but previously unrecognized gaseous environment in the Galactic center, and is beginning to provide unique information on interstellar gas in external galaxies.