Spectroscopic studies of the upper atmospheres of the giant planets using infrared wavelengths sensitive to the H3+ molecular ion show that this species plays a critical role in determining the physical conditions there. For Jupiter, we propose that the recently detected H3+ electrojet holds the key to the mechanism by which the equatorial plasma sheet is kept in (partial) co–rotation with the planet, and that this mechanism also provides a previously unconsidered source of energy that helps explain why the jovian thermosphere is considerably hotter than expected. For Saturn, we show that the H3+ auroral emission is ca. 1% of that of Jupiter because of the lower ionospheric/thermospheric temperature and the lower flux of ionizing particles precipitated there; it is probably unnecessary to invoke additional chemistry in the auroral/polar regions. For Uranus, we report further evidence that its emission intensity is controlled by the cycle of solar activity. And we propose that H3+ emission may just be detectable using current technology from some of the giant extra–solar planets that have been detected orbiting nearby stars, such as Tau Bootes.